Seed Admission Webhook preventing accidental deletion of extension CR(D)s

[rfranzke]

What would you like to be added:
Let's add a new separate binary to g/g that serves a validation webhook in the seed clusters in order to prevent accidental deletion of extension CR(D)s. It shall only allow the deletion if it was confirmed before (similar to how shoot/project validation is prevented).

Why is this needed:
Deleting an extension CRD can have catastrophic and hard-to-recover consequences, hence, let's protect them from accidental deletion.