Add maintains login in mixed env spec

Description of the test

Adds the first basic mixed environment test. Simply validates that when a user is redirected from a canary to a non-canary node that their login session is maintained.

This test targets Staging currently. After proven, will expand to production.

It is tagged as mixed_env in RSpec metadata.

Related to: #338970 (closed)

Dependencies

This spec requires the following additional MR's to be merged first:

• !69421 (merged)
• gitlab-org/quality/pipeline-common!90 (merged)

Check-list

• Confirm the test has a testcase: tag linking to an existing test case in the test case project.
• Note if the test is intended to run in specific scenarios. If a scenario is new, add a link to the MR that adds the new scenario.
• Follow the end-to-end tests style guide and best practices.
• Use the appropriate RSpec metadata tag(s).
• Ensure that a created resource is removed after test execution. A Group resource can be shared between multiple tests. Do not remove it unless it has a unique path. Note that we have a cleanup job that periodically removes groups under gitlab-qa-sandbox-group.
• Ensure that no transient bugs are hidden accidentally due to the usage of waits and reloads.
• Verify the tags to ensure it runs on the desired test environments.
• If this MR has a dependency on another MR, such as a GitLab QA MR, specify the order in which the MRs should be merged.
• (If applicable) Create a follow-up issue to document the special setup necessary to run the test: ISSUE_LINK
• If the test requires an admin's personal access token, ensure that the test passes on your local with and without the GITLAB_QA_ADMIN_ACCESS_TOKEN provided.