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>

protobuf/plugin/doc.go

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+package plugin