Support keycloak custom format via configuration (backport #13216)

[mergify]

Proposed Changes

Implements feature #12324

Docs pr:

• Support keycloak custom format via configuration rabbitmq-website#2185

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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This is an automatic backport of pull request #13216 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of bf7de92 has failed:

On branch mergify/bp/v4.1.x/pr-13216
Your branch is ahead of 'origin/v4.1.x' by 3 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit bf7de92aa.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbitmq_mqtt/test/auth_SUITE.erl

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of aeda3ca has failed:

On branch mergify/bp/v4.1.x/pr-13216
Your branch is ahead of 'origin/v4.1.x' by 4 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit aeda3cada.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbitmq_mqtt/test/auth_SUITE.erl

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally