Floss: Add macro to generate D-Bus interface client

This adds macro to generate D-Bus client method calls. Callback param
type is not yet supported and can be written manually. Future work will
add support to it.

Bug: 200732080
Tag: #floss
Test: Build floss on Linux and test btclient manually

Change-Id: If14a35e335f2370fe60b074f82015a95a9101d2f