le_audio: Improve LeAudioLtvMap

This adds the .Append(other) functionality for combining multiple LTV maps,
and another flavor of RawPacket() serialising function which does the
automatic buffer allocation.

This will be used in the following patches.

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

Change-Id: I6e0478b9d3be130e2b6ac38a597767668f87f849