rust/bt-broadcast-assistant: Add debug logging

Add debug! and trace! logging throughout Broadcast Assistant,
delegator Peer wrapper, and BASS client to improve visibility into
broadcast source discovery, delegator connection, and BASS operations:
 - In assistant: log starting event stream, delegator scanning,
   delegator connection, BASS service discovery, and force-discovery
   calls.
 - In peer: log BASS client wrapper initialization, adding broadcast
   sources, updating synchronization, removing sources, sending
   broadcast codes, informing remote scan state, and processing BASS
   notification events.
 - In bt-bass: log BASS client characteristic discovery, BASS
   notifications on characteristic handles, and updated receive state.
 - Add unit tests in debug.rs verifying AssistantCmd::Verbose verbosity
   updates and scoped command execution with -v and -vv flags.

Test: cargo test -p bt-broadcast-assistant -p bt-bass
      cargo test --workspace
Change-Id: I9c4e875bd172feec7aaf2861c934e05d6a6a6964
Reviewed-on: https://bluetooth-review.googlesource.com/c/bluetooth/+/4122