askrene-bias-channel: bias call add up.

[Lagrang3]

The channel bias feature is not being used yet by any plugin, so this hopefully doesn't break any working code.
When askrene-bias-channel is called the bias quantity is added on top of any previous biased already present on that channel instead of overwriting it.

Changelog-Changed: askrene-bias-channel: bias call add up.

The use case I envision is the following in the context of a payment plugin:

Stack biases on channels that fail often with weird errors and avoid DoS:

• plugin sends payment for computed route
• route fails with weird error such as WIRE_AMOUNT_BELOW_MINIMUM indicating that we need to update the gossmap
• we don't have to trust this channel is behaving correctly, so we update the gossmap and we add some negative bias to it.
• next time we fail the payment again with the same channel we add more bias to it. Eventually this channel will be completely avoided.

[rustyrussell]

But you can't do this atomically. So if you have a system where you really want to control biases in absolute terms, this is bad. For example, you might set it absolutely based on timing detection (i.e. bias against slow channels).

So should we add a new parameter "relative" (bool, default false) which makes it cumulative like this?

[Lagrang3]

OK. A boolean relative flag would help having both use cases.