feat: use Client Credentials for managing Keycloak Clients

[slaskawi]

Description

Please do not review - it's work in progress!

Still to be done:

• Enable Client Policy and Fine-grained Authorization, so that Pepr could only see its own Clients
• Tests for Client Credentials grant in UDS Identity Config
• Handling of the Client Secret rotation
• Update the logic in Pepr to leverage 3 states - old, new and auto
• Document the Realm upgrade (docs, a video and maybe an OpenTofu file)
• Document Pepr logic for picking the implementation

Related Issue

Fixes #1330

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Steps to Validate

• If this PR introduces new functionality to UDS Core or addresses a bug, please document the steps to test the changes.

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide followed