add a function to modify localmods

[Lagrang3]

Function gossmap_local_updatechan can be used to add changes to the gossmap that will apply only locally/temporarily. The function takes as arguments all fields inside a half_chan.
The problem is that there is no API to get the current values inside a gossmap_localmods. Therefore there is no way to update for instance a single field inside one specific localmod without overwritting the other properties.

In this PR I add a new function gossmap_local_updatechan_selected that updates the localmod by changing only selected fields passed as arguments, null values will be ignored. If the localmod does not exist for that scid, it will fetch the original channel values, if the localmod already exists it will fetch those channel values instead, then it will modify the selected fields only.

As a result we can have a localmod API that can stack property modifications on several calls. For instance:

gossmap_local_addchan(...); // create a localmod
gossmap_local_updatechan(...); // update all properties 
gossmap_local_updatechan_selected(..., enabled=false); // make sure enable is false, keep the rest as it is
gossmap_local_updatechan_selected(..., base_fee = 0); // make sure base_fee is 0, keep the rest as it is

[rustyrussell]

I'm a bit confused? The current values are all available in the struct gossmap_chan for the localmod? This means you can build such an API?

[Lagrang3]

@rustyrussell, anyone can read a gossmap_chan data and produce a localmod with gossmap_local_updatechan, but I don't see how you can add an update to a localmod. There is no public interface to get the values of a localmod.
struct localmod, struct gossmap_localmods and find_localmod are hidden.