Add gRPC test server and client

[grcevski]

This PR adds a simple gRPC test and client for testing the implementation of our eBPF instrumentation with gRPC.

Few features:

• The server and client support both unencrypted and TLS mode.
• The client doesn't verify the TLS certificate for simplicity purposes.
• The client runs few different RPC calls, streaming included. Simple single RPC call can be made by adding the -ping command line option on the client command line.

Closes #43

[mariomac]

👏🏻

[grcevski]

Thanks Mario!