Backend: Move Gitlab::HTTP to gems

As a part of the "moving libs to gems" effort (!122915 (merged)), in this issue, we aim to move lib/gitlab/http.rb to the gems as a monorepo gem.

Work list:

• Initial implementation: !125024 (merged)
• Documentation change (https://docs.gitlab.com/ee/development/secure_coding_guidelines.html#gitlab-http-library).
• .gitlab/CODEOWNERS change. (!125024 (comment 1544693859))
• As a migration step, use the new gem inside of lib/gitlab/http.rb.
• Move config.http_proxy_env = Gitlab.http_proxy_env? to the gem (!125024 (comment 1506502552))
• Finally, replace or remove the require_relative in config/initializers/gitlab_http.rb. See: !125024 (comment 1502698924)
• Handle silent mode !125024 (comment 1511792885)
• Use extra_allowed_uris instead of allowed_internal_uris !125024 (comment 1512071587)
• Handle Rubocop TODOs
• spec.email = ["engineering@gitlab.com"] !125024 (comment 1555514248) -> #424940 (closed).
• the feature flag use_gitlab_http_v2 #426549 (closed).

Implementation steps:

1. Create gitlab-http gem (!125024 - merged)
2. Fix some rubocops for gitlab-http and fix the e... (!131612 - merged)
3. Enable gitlab-http gem and its patches (!132238 - merged)
4. Replace UrlBlocker::BlockedUrlError with HTTP_V2 (!132541 - merged)
5. Use the HTTP_V2 alongside LegacyHTTP (!132742 - merged)
6. Modify gitlab-http docs and add CODEOWNERS (!133687 - merged)
7. Add async support to gitlab-http (!135127 - merged)