Adding askrene-disable-channel

[Lagrang3]

I can think of a way to effectively disable a channel in askrene: by calling askrene-inform-channel and set the maximum_msat to 0. But it seems like a workaround way. I think since we already have an askrene-disable-node command, we should also
support an askrene-disable-channel command as well.

I have also refactored the way channels are marked as disabled.
Previously we had disabled channels by using gossmap_local_updatechan call which would produce a plugin crash
when calling getroutes if we disable a channel that either doesn't exist or it is defined in a layer that we don't use
in that call. Now instead we build a bitmap of disabled channels that we pass on to the MCF solver. The bitmap uses the indexes of the channels relative to the final gossmap, ie. the public gossip plus all localmods coming from the layers.

[ShahanaFarooqui]

@Lagrang3 I have updated the third commit (d4cb5b8) for addressing the CI failures in the Pre-build checks, which were caused by missing updates to schema.json and index.rst.

[rustyrussell]

Definitely need this interface, I agree.

But I prefer to fix the gossmap_update_localchan semantics, otherwise there is a potential hole if we apply a modification and then the channel in the underlying gossip_store is removed: we would have the same crash.

Also, we should not combine nodes and channels in the report output: harsh experience has lead us to now avoid things which don't have simply types (grpc interface has strong types, making this awkward).

[rustyrussell]

Rolled into #7707