Migrate Macro Annotations

[juanpedromoreno]

As part of the #290 plan, we need to migrate the current Scalameta macros to the new generation of Macro Annotations.

The current scalameta annotations are:

Annotation	Scope	Arguments	Description
@service	Trait	-	Tags the trait as an [RPC] service, in order to derive server and client code (macro expansion).
@rpc	Method	(SerializationType, Compression)	Indicates that the method is an RPC request. As SerializationType parameter value, Protobuf and Avro are the current supported serialization methods. As Compression parameter value, only Gzip is supported.
@stream	Method	[S <: StreamingType]	Indicates the method's streaming type: server, client, and bidirectional. Hence, the S type parameter can be ResponseStreaming, RequestStreaming, or BidirectionalStreaming, respectively.
@message	Case Class	-	Tags the case class as an RPC message.
@option	Object	(name: String, value: Any)	Defines the equivalent headers in .proto files.
@outputPackage	Object	(value: String)	Defines the package declaration in .proto files, and the namespace tag in .avpr files.
@outputName	Object	(value: String)	Defines the protocol tag in .avpr files; if missing the idlGen tool will use the source file name (without .scala extension) instead.

For all these annotations, the only one that is being actually expanded (in terms of expanded code) is @service. The rest of them can be considered as like "anchors"/"settings" used by the @service annotation.

We could consider doing this work creating smaller issues under the feature-branch feature/scalamacros-migration, for example.

[L-Lavigne]

WIP branch: https://github.com/frees-io/freestyle-rpc/commits/MacroConversionWIP (won't compile idlGen module until #298 is done).

[pepegar]

Done in #328 ! 🎉