csis: Add support for getting list of devices per group

This is needed by native modules which are using CSIS groups for some actions e.g. VC

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
Test: atest --host bluetooth_csis_test

Change-Id: I328cf99f8899760733e4a0ee116c8e5b3dba1ba8