Implement 'Gardener' ClusterProvider

[In-Ko]

Description

For our new MCP system as well as its new setup, we need to be able to order Gardener shoot clusters in an improved way.

We have to implement a operator that fulfils our ClusterProvider contract and is able to manage shoot clusters.

https://github.com/openmcp-project/cluster-provider-gardener

We already have logic for managing Gardener shoot clusters for the current MCP implementation, we can probably re-use some parts of this and don't have to implement it from scratch. Although we can probably improve the implementation here and there.

We should directly introduce a way to overwrite specific fields of the resulting shoot manifest, see task "add possibility for operators to manipulate generated shoot manifest" (ToDo: create task)

Prerequisites: #10

ToDo:

• implement ProviderConfig controller
• implement Cluster controller
• implement AccessRequest controller
• polishing (fix edge cases, improve error handling for shoot watch, etc.)
• add possibility to overwrite shoot fields
• tests
• documentation

[Diaphteiros]

I just noticed that the feature add possibility to overwrite shoot fields is still missing and I'm working on it.

[Diaphteiros]

The missing feature is implemented, currently waiting for a controller-utils release to open the PR.

[Diaphteiros]

PR is open: openmcp-project/cluster-provider-gardener#68

[Diaphteiros]

I did some changes to adapt to our latest discussions (mainly that a Cluster can reference multiple cluster configuration resources) and added them to the aforementioned PR - this task is done, as soon as that PR is merged.