[wip] Xpay limit path length

[Lagrang3]

Addresses: #9156
Supersedes: #9539

This PR adds a new constraint into askrene: maxhops, to limit the length of the routes.
Hard constraints of path length is not possible under the current implementation, minimum cost flows
do not allow that. Our approach is to add a constant cost per hop (a similar tweak in renepay #7540 was already deployed)
and scale that cost until the computed routes fit the maxhops constraints.
Fewer hops are expected with a base cost per hops and a getroutes failure if no routing solutions
are found satisfying maxhops constraint.

On xpay we compute the onion budget for each payment as the maximum number of hops we can afford to put inside
an onion message given the hard limit of 1300 bytes for the onion package, payment amount, blockheight and tail
(ie. final payload in the case of bolt11 and blinded path plus final payload in the case of bolt12).
The onion budget is passed down to getroutes so that solutions will not exceed it.
No more "Could not create payment onion: path too long!" errors are expected afterwards, instead we might
get an error from getroutes if no solutions can be found with the maxhops limit.

TODO:

• unit test onion_budget,
• implement the cost based maxhops constraint into askrene,
• integration test for maxhops in askrene,
• integration test for xpay with heavy final payloads bolt11,
• integration test for xpay with heavy final payloads bolt12,
• promote maxhops from a developer option to a user option and add documentation for maxhops,