bluetooth: tester: Refactor BTP handling

This changes the way of BTP commmand handlers are called. Instead of
calling functions to handle service we now provide API for registering
callbacks per command. This allows to keep common checks (size etc)
in single place for (most) commands. Also provide common buffer for
reponses forcing callbacks to handle replay in common way.

In this commit only Core and GAP services are converted. This leaves
rest of services unfuntional (but code stil compiles correctly).
Other services are converted in following commits.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>