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]

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[mattmoor]

/lifecycle frozen