blip18 inbound fee support

[DerEwige]

At the last specs meeting inbound fees got mentioned again lightning/bolts#1300

LDK signaled that they consider supporting them with the new gossip version.
And @t-bast also sounded more open to it than in the past.

Maybe you could have a look again at: #3126
It is up to date with version 0.13.1 and is working very well for me.

[t-bast]

The spec meeting did not mention adding support for blip 18 to existing implementations. We discussed adding support for some form of inbound fees as part of the taproot gossip feature, which introduces a new channel_update message and thus a good opportunity for a standard form of restricted inbound fees.

The spec for this needs to be added to the taproot gossip PR first. We still don't plan to add support for inbound fees as defined by blip-18.