Support GRPC as transport

Create a module (built-in? external?) that will provide ability to define an GRPC protocol/endpoint that will be served using user defined Lua function(s). I see the following subtasks:

1. Parse and compile a GRPC protocol/endpoint description, which is given as a Lua string (at least assume the compilation step in the API to have ability to implement it later).
2. Parse a GRPC request and validate it against given GRPC protocol/endpoint description. The result of parsing / validation should be either a meaningful error or a request in the Lua format.
3. Provide ability to encode a GRPC response from a Lua value with validation against given protocol/endpoint definition.
4. Consider providing a helper for using the module together with the tarantool/httpng module: a way to acquire a handler for the http server for a particular protocol/endpoint that will be served by user defined Lua function(s).

Many details are to be investigated and defined better. That is just my very high level vision.

BTW, we can implement a protobuf schema / binary format support as a separate module. Also maybe it is good to start from just protobuf, because GRPC is based on it (AFAIK).

The original wording describes various tasks around GRPC: expose iproto via GRPC, expose a user defined endpoint, connect to an external service. We decided to shrink the scope of this task and concentrate of the second point. See the original description below (it can be used as base for future issues).

No files, tests, or entry points are named. Start by reading the narrowed scope in the issue and clarifying the GRPC endpoint, protocol-description, request-validation, and response-encoding requirements. Done means a defined implementation plan and a user-defined Lua-served GRPC endpoint with the required validation and encoding behavior.