webhook binding labels should be more finely grained

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

Start by locating the webhook binding logic and reviewing the annotation examples in this issue. Clarify the per-binding opt-in/out behavior and annotation format, then verify that the webhook handles the listed binding groups accordingly.

Written by the indexing model from the issue text.