operator: Refactor handling of credentials in managed-auth mode

[xperimental]

What this PR does / why we need it:

This PR builds on the integration of token and managed token credentials modes for AWS, Azure and GCP and tries to simplify the approach taken. It combines a few different things, maybe it makes sense to split some of it up into separate PRs:

• Remove separate controller for CredentialsRequest, the resource is now created in the same namespace as the LokiStack simplifying the dependency management
• Fix issue with CredentialsRequest not containing the ruler ServiceAccount
• Reconcile contents of CredentialsRequest on subsequent reconciliation runs (like other resources)
• Show what type of credential (static, token, managed) is used in status of LokiStack
• Use same paths / volume names across the different cloud providers
• Only read environment configuration (for managed auth) once at operator startup

Which issue(s) this PR fixes:

• CredentialsRequest does not contain ruler account (LOG-5061)

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Tests updated
• CHANGELOG.md updated

[periklis]

LGTM