Add btmon BT_DEBUG backend for bsim targets

[aescolar]

Is your enhancement proposal related to a problem? Please describe.

When developing/debugging using bsim targets, it would be nice to be able to monitor the HCI traffic in btmon.

Describe the solution you'd like

Today there is a BT_DEBUG_MONITOR_UART and BT_DEBUG_MONITOR_RTT backend for real targets but not for bsim targets.
Either getting that backend to work for a bsim target or adding one new backend for bsim targets would be nice.

Describe alternatives you've considered

Increasing the amount of logging in the BT_DEBUG_LOG backend, but that would be more tedious that using btmon.

Additional context
Requested by @Thalley
Partially related to #50897

[aescolar]

CC: @jhedberg @cvinayak @Vudentz @thoh-ot

[zephyrbot]

Hi @jhedberg,

This issue, marked as an Enhancement, was opened a while ago and did not get any traction. It was just assigned to you based on the labels. If you don't consider yourself the right person to address this issue, please re-assing it to the right person.

Please take a moment to review if the issue is still relevant to the project. If it is, please provide feedback and direction on how to move forward. If it is not, has already been addressed, is a duplicate, or is no longer relevant, please close it with a short comment explaining the reason.

@aescolar you are also encouraged to help moving this issue forward by providing additional information and confirming this request/issue is still relevant to you.

Thanks!