feature: Add admission webhook for defaulting APIExport's path

[xmudrii]

Feature Description

When referencing APIExport in FilteredAPIExportEndpointSlice, path should be optional. If it's not provided, it should default to the current workspace/logicalcluster. This needs to be done in an admission webhook that's registered in kcp.

Proposed Solution

Create an admission webhook that's registered in kcp to handle this.

Alternative Solutions

The initial idea was to use admission plugin in this Virtual Workspace. However, this can't work because FilteredAPIExportEndpointSlice are not created via the virtual workspace, they're created in kcp directly, so admission webhook needs to be on the kcp level. Such plugin was implemented, and it potentially can be take as a starting for building the webhook: https://gist.github.com/xmudrii/36c358478622ca077e8e30516024c7a6

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Additional Context

No response