events: autogenerate fieldpath filters

To ensure consistent fieldpath matching for events, we generate the
fieldpath matching using protobuf definitions. This is done through a
plugin called "fieldpath" that defines a `Field` method for each type
with the plugin enabled. Generated code handles top-level envelope
fields, as well as deferred serialization for matching any types.

In practice, this means that we can cheaply match events on `topic` and
`namespace`. If we want to match on attributes within the event, we can
use the `event` prefix to address these fields. For example, the
following will match all envelopes that have a field named
`container_id` that has the value `testing`:

```
ctr events "event.container_id==testing"
```

The above will decode the underlying event and check that particular
field. Accordingly, if only `topic` or `namespace` is used, the event
will not be decoded and only match on the envelope.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

events/exchange.go

@@ -219,25 +219,8 @@ func validateEnvelope(envelope *events.Envelope) error {
 }
 
 func adapt(ev interface{}) filters.Adaptor {
-	switch ev := ev.(type) {
-	case *events.Envelope:
-		return filters.AdapterFunc(func(fieldpath []string) (string, bool) {
-			if len(fieldpath) == 0 {
-				return "", false
-			}
-
-			switch fieldpath[0] {
-			case "namespace":
-				return ev.Namespace, len(ev.Namespace) > 0
-			case "topic":
-				return ev.Topic, len(ev.Topic) > 0
-			default:
-				// TODO(stevvooe): Handle event fields.
-				return "", false
-			}
-		})
-	case filters.Adaptor:
-		return ev
+	if adaptor, ok := ev.(filters.Adaptor); ok {
+		return adaptor
 	}
 
 	return filters.AdapterFunc(func(fieldpath []string) (string, bool) {