Generate webhook functions for fields with `+immutable` comment tag

[muvaf]

What problem are you facing?

Webhooks are available in Crossplane now and immutability check on UPDATE action is implemented manually for each field.

How could Crossplane help solve your problem?

Similar to reference resolver generation, we can have a generator in crossplane-tools that will scan the fields for // +immutable tag and generate the logic automatically.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[negz]

We might also be able to do this with CEL now per kubernetes/kubernetes#65973 (comment). See also kubernetes/website#36032.