Updates plugin and client packages

Updates plugin package. Removes grpc server actions from the package
since this is being moved to a new plugin-lib repo.

Updates client package to use new function prototypes of passing metric
all the way through a workflow instead of bytes through process/publish.

For native/gob, the encoding/decoding now happens in the client. This is
done because the concept of contentType was removed form the framework
but we still want to support existing plugins. Doing this in the client
allows existing plugins to interact with plugins written using any other
langauge lib.

Updates proto definitions, including adding some types that were
previously referenced from common. This was done to allow plugin-libs
written in other languages to only require one proto file to generate
from, reducing complexity for the plugin-lib authors at the cost of some
duplication.

control/plugin/client/client.go

@@ -44,11 +44,11 @@ type PluginCollectorClient interface {
 // PluginProcessorClient A client providing processor specific plugin method calls.
 type PluginProcessorClient interface {
 	PluginClient
-	Process(contentType string, content []byte, config map[string]ctypes.ConfigValue) (string, []byte, error)
+	Process([]core.Metric, map[string]ctypes.ConfigValue) ([]core.Metric, error)
 }
 
 // PluginPublisherClient A client providing publishing specific plugin method calls.
 type PluginPublisherClient interface {
 	PluginClient
-	Publish(contentType string, content []byte, config map[string]ctypes.ConfigValue) error
+	Publish([]core.Metric, map[string]ctypes.ConfigValue) error
 }