Add routehints with 2 levels deep hophints for Lightning Box

[hsjoberg]

This should use the liquidity querying API to Dunder in order to create routehints with hophints that we know have Liquidity.
It could potentially increase payments success.

As this is with LNURL-pay, the invoice size does not matter.

Note: should be made into a LN P2P message bLIP in the future.