Extract stub_settings_source into spec support file

What does this MR do and why?

This MR extracts the RSpec trait stubbing_settings_source into a separate support file.

It also renames stubbing_settings_source to stub_settings_source to keep consistency across other support files.

We need to require this support file in spec where stub_settings_source: true is used.

This avoids order dependent flaky specs like:

bin/rspec spec/commands/sidekiq_cluster/cli_spec.rb spec/lib/gitlab/pagination/offset_header_builder_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
.....................................bundle exec sidekiq -c2 -etest -t25 -gqueues:foo -r/home/peter/devel/gitlab/gdk-ee/gitlab -qfoo,1
.........FFF

Failures:

  1) Gitlab::Pagination::OffsetHeaderBuilder#execute sets headers to request context
     Failure/Error: uri.host = Gitlab.config.gitlab.host

     Settingslogic::MissingSetting:
       Missing setting 'gitlab' in /tmp/gitlab.yml20220120-2766343-4qkoh2
     # ./lib/gitlab/pagination/offset_header_builder.rb:48:in `block in base_request_uri'
     # ./lib/gitlab/pagination/offset_header_builder.rb:47:in `tap'
     # ./lib/gitlab/pagination/offset_header_builder.rb:47:in `base_request_uri'
     # ./lib/gitlab/pagination/offset_header_builder.rb:54:in `build_page_url'
     # ./lib/gitlab/pagination/offset_header_builder.rb:62:in `page_href'
     # ./lib/gitlab/pagination/offset_header_builder.rb:38:in `block in pagination_links'
     # ./lib/gitlab/pagination/offset_header_builder.rb:37:in `tap'
     # ./lib/gitlab/pagination/offset_header_builder.rb:37:in `pagination_links'
     # ./lib/gitlab/pagination/offset_header_builder.rb:26:in `execute'
     # ./spec/lib/gitlab/pagination/offset_header_builder_spec.rb:38:in `block (3 levels) in <main>'

  2) Gitlab::Pagination::OffsetHeaderBuilder#execute exclude total headers does not set total headers to request context
     Failure/Error: uri.host = Gitlab.config.gitlab.host

     Settingslogic::MissingSetting:
       Missing setting 'gitlab' in /tmp/gitlab.yml20220120-2766343-4qkoh2
     # ./lib/gitlab/pagination/offset_header_builder.rb:48:in `block in base_request_uri'
     # ./lib/gitlab/pagination/offset_header_builder.rb:47:in `tap'
     # ./lib/gitlab/pagination/offset_header_builder.rb:47:in `base_request_uri'
     # ./lib/gitlab/pagination/offset_header_builder.rb:54:in `build_page_url'
     # ./lib/gitlab/pagination/offset_header_builder.rb:62:in `page_href'
     # ./lib/gitlab/pagination/offset_header_builder.rb:38:in `block in pagination_links'
     # ./lib/gitlab/pagination/offset_header_builder.rb:37:in `tap'
     # ./lib/gitlab/pagination/offset_header_builder.rb:37:in `pagination_links'
     # ./lib/gitlab/pagination/offset_header_builder.rb:26:in `execute'
     # ./spec/lib/gitlab/pagination/offset_header_builder_spec.rb:46:in `block (4 levels) in <main>'

  3) Gitlab::Pagination::OffsetHeaderBuilder#execute pass data without counts does not set total headers to request context
     Failure/Error: uri.host = Gitlab.config.gitlab.host

     Settingslogic::MissingSetting:
       Missing setting 'gitlab' in /tmp/gitlab.yml20220120-2766343-4qkoh2
     # ./lib/gitlab/pagination/offset_header_builder.rb:48:in `block in base_request_uri'
     # ./lib/gitlab/pagination/offset_header_builder.rb:47:in `tap'
     # ./lib/gitlab/pagination/offset_header_builder.rb:47:in `base_request_uri'
     # ./lib/gitlab/pagination/offset_header_builder.rb:54:in `build_page_url'
     # ./lib/gitlab/pagination/offset_header_builder.rb:62:in `page_href'
     # ./lib/gitlab/pagination/offset_header_builder.rb:38:in `block in pagination_links'
     # ./lib/gitlab/pagination/offset_header_builder.rb:37:in `tap'
     # ./lib/gitlab/pagination/offset_header_builder.rb:37:in `pagination_links'
     # ./lib/gitlab/pagination/offset_header_builder.rb:26:in `execute'
     # ./spec/lib/gitlab/pagination/offset_header_builder_spec.rb:57:in `block (4 levels) in <main>'

Finished in 30.8 seconds (files took 3.61 seconds to load)
49 examples, 3 failures

Failed examples:

rspec ./spec/lib/gitlab/pagination/offset_header_builder_spec.rb:33 # Gitlab::Pagination::OffsetHeaderBuilder#execute sets headers to request context
rspec ./spec/lib/gitlab/pagination/offset_header_builder_spec.rb:42 # Gitlab::Pagination::OffsetHeaderBuilder#execute exclude total headers does not set total headers to request context
rspec ./spec/lib/gitlab/pagination/offset_header_builder_spec.rb:53 # Gitlab::Pagination::OffsetHeaderBuilder#execute pass data without counts does not set total headers to request context

Note, both specs about are run with fast_spec_helper so previously they would fail because RSpec trait stubbing_settings_source was not available.

Contributes to #350739.

How to set up and validate locally

Run:

bin/rspec spec/commands/sidekiq_cluster/cli_spec.rb spec/lib/gitlab/pagination/offset_header_builder_spec.rb

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.