feat: Improve clusteraccess

[Diaphteiros]

What this PR does / why we need it:
See release notes.

Which issue(s) this PR fixes:
Required for unit tests belonging to openmcp-project/backlog#95

Special notes for your reviewer:

Release note:

The `StableRequestName` and `StableRequestNameFromLocalName` functions from the `lib/clusteraccess` package now shorten resulting names if they exceed the character limit for k8s resource names. A hash suffix prevents conflicts for different keys with the same prefix.

The `NewTestClusterAccessManager` function from `lib/accessrequest` serves as an implementation of the `Manager` interface that is suited to be used in unit tests. It fakes readiness of `ClusterRequest` and `AccessRequest` resources that it creates and returns k8s fake clients instead. The fake clients have to be configured before. While this makes the `Manager` usable in unit tests, the code creating the `Manager` still has to differentiate between the creation of a regular one or the test implementation.