make it easier to import the crdpuller from other projects

[xrstf]

Summary

The api-syncagent imports the crdpuller. Today I tried updating its dependencies to the latest multicluster-runtime, but this would bring the agent to Kubernetes 1.35. This then clashes with the import to kcp when running go mod tidy:

go: github.com/kcp-dev/api-syncagent/internal/discovery imports
        github.com/kcp-dev/kcp/pkg/crdpuller imports
        k8s.io/apiextensions-apiserver/pkg/apiserver imports
        k8s.io/apiserver/pkg/server imports
        k8s.io/apiserver/pkg/admission/plugin/namespace/lifecycle imports
        github.com/kcp-dev/client-go/informers imports
        github.com/kcp-dev/client-go/informers/storagemigration imports
        github.com/kcp-dev/client-go/informers/storagemigration/v1alpha1 imports
        k8s.io/client-go/listers/storagemigration/v1alpha1: module k8s.io/client-go@latest found (v0.35.2), but does not contain package k8s.io/client-go/listers/storagemigration/v1alpha1

For testing, I copied the entire crdpuller package into the syncagent and just removed the line about filtering resources part of apiextensions-apiserver. This solved the dependency hell. Therefore I think it's worth it to introduce a small indirection in kcp: instead of hardcoding a specific scheme in the crdpuller to ignore, it can now be configured by the caller, and in a separate package we provide the default scheme to ignore.

I provided a public MergeSchemes func to aid people in constructing their own scheme to ignore.

What Type of PR Is This?

/kind cleanup

Release Notes

NONE

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[xrstf]

part of kcp-dev/api-syncagent#146