pay: Share channel_hints across payments and plugins

[cdecker]

While performing a payment we attempt to get several HTLCs through
from the sender to the destination. Some of these attempts may fail,
due to temporary unavailability, insufficient balance, or many other
reasons. We use channel_hints to track these failures and compile
the observations into a more detailed view of the network.

So far we were just forgetting all of the inferred information once
the payment is done, however this information is useful for later
payments too, as it allows us to skip attempts that are unlikely to
succeed, and essentially continue the exploration where the previous
payments have left off.

The main issue we face is how do we relax the hints over time, such
that we maximize their usefulness by allowing us to skip channels that
are unlikely to succeed, while at the same time minimizing the false
negative rate where we skip a channel that would have worked, based on
the prior observations. We implement it here as a linear decay, in
line with the leaky bucket rate limiting algorithm.

We model the sharing of the channel_hints as notifications since we
might want to share observations across multiple plugins, and maybe
even across multiple nodes, if they are operated by the same
operator. DO NOT accept channel_hint notifications from untrusted
sources, as they can be used directly to steer the route selection.

Changelog-Added: pay: Payments now emit channel_hint_updated notification to share inferred balances and observations across multiple payments.

This is part of the following PR set:

graph TD;
    CLN7487 --> CLN7494;
    click CLN7487 "https://github.com/ElementsProject/lightning/pull/7487"
    click CLN7494 "https://github.com/ElementsProject/lightning/pull/7494"
    style CLN7487 fill:#AAFFAA

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