Run RSpec full jobs when `pipeline:run-full-rspec` label is added

What does this MR do and why?

This MR adds a rule in pipeline configuration to run rspec full jobs when ~"pipeline:run-full-rspec" label is added to the merge request by @gitlab-bot on approval.

~"pipeline:run-full-rspec" label will be added by @gitlab-bot on merge request's approval: gitlab-org/quality/triage-ops!1666 (merged)

By default, the pipeline runs rspec minimal jobs.

Testing

Tested changes in !98497 (closed)

header	Result
backend file change (Tested by setting $CI_MERGE_REQUEST_APPROVED false 9f02e8c8)	✅ rspec minimal jobs have been triggered: https://gitlab.com/gitlab-org/gitlab/-/pipelines/646248315
with ~"pipeline:run-full-rspec" label	✅ rspec full jobs have been triggered: https://gitlab.com/gitlab-org/gitlab/-/pipelines/646267866

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #373064 (closed)