askrene: add layer auto.ignorelocalhtlclimits

[Lagrang3]

The new layer "auto.ignorelocalhtlclimits" helps remove the HTLC_min/max limits on local channels.

In renepay we already ignore those limits, see PR #7159.

[rustyrussell]

First, let me start by saying this implementation is perfect 🧡 Documentation, tests, clean implementation...

However, there are real constraints on local capacities. injectpaymentonion just uses the same logic as forwarding (which has the advantage of consistency, so we're not leaking information about the source of payment). We could loosen that if we want to, but the remote side also gives us a max_htlc_value_in_flight_msat (which is a total for all htlcs, but logically an upper bound on any single one) and a htlc_minimum_msat which we are not allowed to go beneath.

So if we were to do this, we should use those constraints, which is a bit trickier....

[Lagrang3]

Good point!
As a matter of fact we have just added in v25.02 this new parameter their_max_htlc_value_in_flight_msat,
let's see how we can use it to constraint the sum of all pending HTLCs.

On the leak side... I am not sure, we would be revealing our immediate peer that we are the source of the payment,
he already knows how we own on that channel.
But yes, we probably shouldn't let him also know that we are the source of the payment.