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>

Protobuild.toml

@@ -1,6 +1,6 @@
 version = "unstable"
 generator = "gogoctrd"
-plugins = ["grpc"]
+plugins = ["grpc", "fieldpath"]
 
 # Control protoc include paths. Below are usually some good defaults, but feel
 # free to try it without them if it works for your project.