webhook binding labels should be more finely grained

[lberk]

As @mattmoor described in #1208, we face a growing number of bindings.

I'd left a TODO about adding per-Binding opt-in/out, so I wonder if we're going to go this route whether we should be doing so using a fine-grained annotation like that instead.. WDYT?

Could be something like: {gvr}/binding: enabled? so:

sinkbindings.sources.knative.dev/binding: "enabled"
githubbindings.bindings.knative.dev/binding: "enabled"
vspherebindings.sources.tanzu.vmware.com: "enabled"
...

the webhook should implement this

[github-actions[bot]]

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.

[mattmoor]

/lifecycle frozen