Add validation webhook for Trigger.

[vaikas]

It should disallow some changes because the underlying resources are immutable, and the only way to update them is to delete/recreate. For example, changing a Delivery.DeadLetterSink results possibly in creation/deletion of queues and will create dropped events which is probably unexpected from the users POV.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[xtreme-sameer-vohra]

/remove-lifecycle stale

[embano1]

We should also add validation around delivery settings, e.g. retries.

cc/ @benmoss

[benmoss]

/assign

[benmoss]

I'm not sure how changing the DLQ would result in dropped events. It seems like we'd stop one dispatcher and create a new one on the same queue. I created #465 since we don't handle changing the DLQ very well right now