✨ Add Gardener provider

[rfranzke]

Follow-up of 063ede9

This PR introduces a minimal Gardener provider for the multicluster-runtime project, enabling integration with Gardener's multi-cluster management. The provider supports two flavors:

• garden Flavor: Controller interacts with the garden cluster, watches Shoot resources, and uses short-lived, auto-renewed admin kubeconfigs.
• seed Flavor: Controller connects to a seed cluster, watches Cluster resources, and leverages the gardenlet's short-lived, auto-renewed cluster-admin kubeconfig.

Similar to #7

[k8s-ci-robot]

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[xrstf]

I think a better place for such implementations would be a repository in the Gardener GitHub organization, rather than here. This would simplify the PR reviews as Gardener folks could review their own stuff in their own repository, rather than having to contribute to one shared "village bicycle" repository here.

I don't think this repo here wants to become a monorepo holding various implementations from various authors at varying quality levels. That would create a sort of expectation that all providers in this repo are at least somewhat maintained, and the maintainers here simply cannot provide that guarantee.

[rfranzke]

Good points @xrstf! @embik and @mjudeikis shared similar thoughts. Let's
/close
this PR accordingly. Perhaps,

multicluster-runtime/README.md

Line 47 in 7e76384

6. multicluster-runtime is provider agnostic, but may contain providers with its own go.mod files and dedicated OWNERS files.

should be adapted to prevent others from proposing similar providers here :)

[k8s-ci-robot]

@rfranzke: Closed this PR.

In response to this:

Good points @xrstf! @embik and @mjudeikis shared similar thoughts. Let's
/close
this PR accordingly. Perhaps,

multicluster-runtime/README.md

Line 47 in 7e76384

6. multicluster-runtime is provider agnostic, but may contain providers with its own go.mod files and dedicated OWNERS files.

should be adapted to prevent others from proposing similar providers here :)

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[xrstf]

Perhaps, [...] should be adapted to prevent others from proposing similar providers here :)

Absolutely agree on that point :-)