Add support for receiving Bolt 12 payments without plugins

[t-bast]

Our release v0.11.0 included official support for Bolt 12, but we currently need a plugin to allow receiving Bolt 12 payments (e.g. https://github.com/ACINQ/eclair-plugins/tree/master/bolt12-tip-jar). While this is indeed important for users who want to carefully control their blinded paths to guarantee privacy, we should also offer a basic mode where we take care of this without plugins.

We should expose:

• an API to create offers
• configuration options to help a basic blinded path selection algorithm

I don't know at that point what that will look like, this should be brainstormed!