Add gRPC Activator

[sleipnir]

It would be nice if this elixir-grpc/grpc#254 was implemented before we start this item

We need to dynamically implement a grpc contract, just like we did in the Massa project, for the client for this we need at runtime to compile the proto files via FileDescriptor

Example of converting protobuf json to elixir (pseudo code):

actor = Eigr.Functions.Protocol.Actors.Actor.new(name: "Joe")
{:ok, json} = Protobuf.JSON.to_encodable(actor)
decoded_actor = Protobuf.JSON.decode(json, Eigr.Functions.Protocol.Actors.Actor)

Links:
https://github.com/eigr/massa/blob/379828098fbbb7298de5ebd8ee1525c749555bbd/apps/massa_proxy/lib/massa_proxy/server/grpc_server.ex#L18
https://github.com/eigr/massa/blob/379828098fbbb7298de5ebd8ee1525c749555bbd/apps/massa_proxy/lib/massa_proxy/server/grpc_server.ex#L32
https://github.com/eigr/massa/blob/379828098fbbb7298de5ebd8ee1525c749555bbd/apps/massa_proxy/lib/massa_proxy/util.ex#L155

[sleipnir]

With the possibility of transcoding grpc to http (elixir-grpc/grpc#273) maybe we can solve the item #31 through this one.

[sleipnir]

When completes #302 closes this issue without the need for an Activator