btm: Initial Iso Manager implementation

This adds the ISO Manager used to manage the CIG/CIS and
in future patches also BIG/BIS channels. It defines an API
for the user to call and callback interfaces that the user
has to implement and register in the ISO Manager to use it.

Bug: 150670922
Tag: #feature
Test: atest --host net_test_btm_iso
Sponsor: jpawlowski@
Change-Id: I0f4719b6efb74a7350cf3d6d9263bf855d60ce6f