examples: Add gRPC streaming example

[radeksimko]

Closes #158

[SarahFrench]

I figured this example is ok to merge, given that we've closed out Read/Write in core. I've got some suggestions for the README but it's not a blocker

[SarahFrench]

Suggested change

	This example builds a plugin & client which can stream larger amount of data
	between them while staying below reasonable message size limits of the gRPC
	protocol.
	This example builds a plugin & client which can stream a large amount of data
	between them while staying below reasonable message size limits of the gRPC
	protocol.
	>Note: [hashicorp/go-plugin sets an upper limit on message size](https://github.com/hashicorp/go-plugin/blob/d0d30899ca2d91b0869cb73db95afca180e769cf/grpc_client.go#L39-L41). At time of writing, that value is `math.MaxInt32` bytes, or approximately 2GB.

[SarahFrench]

Suggested change

	Build the plugin
	Navigate into this example directory
	\```
	cd examples/streaming
	\```
	Next, build the streamer plugin:

[SarahFrench]

I don't know how the ``` will behave if the suggestion is accepted. I think the slash will be present in the eventual markdown.

[SarahFrench]

Suggested change

	launch client
	Finally launch the client:

[SarahFrench]

Suggested change

	## To re-generate protobuf definitions
	The client will first write data to the steamer plugin, and then the client will read that
	data back from the plugin. The plugin writes the data it receives in a file called `myfile`,
	due to the argument passed to the client above.
	## To re-generate protobuf definitions

[SarahFrench]

Ah of course we need review from the codeowner