[Feature request] Proximity suppress for repeaters (prox CLI)

[lil-jimmy-93]

I'd like to contribute a "proximity suppress" feature for repeaters.

Use case 1:
For mobile repeaters when a flood packet is heard via a listed nearby
repeater (by pubkey prefix) at/above an SNR threshold, hold flood
retransmits for a hold time so overlapping coverage doesn't add
redundant hops. Direct (DM) packets are not suppressed.

Use case 2:
Poor man repeater HA for remote/hard to get to locations. Install two
repeaters, set the standby repeater up with proximity suppress. If the
primary repeater fails to transmit, the backup automatically comes
online.

Proposed CLI:
set prox on|off
set prox.snr
set prox.hold
set prox.list [,hex...]
get prox

I already have a working implementation based on that design and can
open a PR against dev if this approach looks okay.

I built this for my mobile repeaters, so they weren't adding more airtime
to the mesh when they are parked at home.

Looking for a rough 👍 before submitting code. After this post, I have no
further GitHub experience, so working off of googling and will stumble
along.