Blueprint to support multiple databases

What does this MR do?

This MR documents the architecture blueprint for supporting multiple databases in Omnibus GitLab.

Blueprint to support multiple databases
    
Documents the implementation model when a new component requires its own
database and the requirements for each level of support that Omnibus
GitLab may provide.
    
- Describes the standard when an existing component is migrated to the
  new implementation model.
- Outlines the way a new component with a separate database should be
  added to Omnibus GitLab.
- Defines each level of support a component may have from Omnibus GitLab.

Closes https://gitlab.com/gitlab-org/distribution/team-tasks/-/issues/1349

Signed-off-by: Hossein Pursultani <hpursultani@gitlab.com>

Objectives

• Offering higher levels of support for existing and new components with database requirements.
• Maintaining the existing model of user settings, i.e. gitlab.rb, while changing the underlying implementation where needed.
• Defining a consistent, testable, and extensible architecture and implementation model for supporting multiple databases without imposing breaking changes or forcing fundamental refactoring into Omnibus GitLab.
• Gradually bridging the gap between the existing and new implementation models.

Related issues

Closes gitlab-org/distribution/team-tasks#1349 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.