Add faster login mechanism for end-to-end tests

What does this MR do?

With this mechanism, it is possible to log into GitLab passing secrets as query strings of the /users/qa_sign_in relative URL.

For more details, see the following comment on the original issue #27271 (comment 309547672).

Note: before merging this MR, we need to have the following issue closed https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9897.

Related MRs: gitlab-qa!429 (merged) and omnibus-gitlab!4119 (merged).

Closes #27271.

Running the updated end-to-end test

Use the following command to run a test that uses the formless login mechanism GITLAB_QA_FORMLESS_LOGIN_TOKEN=some-super-secret-token bundle exec bin/qa Test::Instance::All http://localhost:3000 -- qa/specs/features/browser_ui/2_plan/issue/mentions_spec.rb

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team