tests: bluetooth: tester: Add support for L2CAP channels options

This allows for better control over IUT behaviour by Upper Tester.

PTS and TS require inconsistent behaviour in terms of how IUT should
return credits. Some tests require return on explicit UT request and
some require that IUT returns credits autonomously.

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