initial support bolt12 offers

[accumulator]

This PR contains initial support for BOLT12

• parses bolt12 offer payment identifiers,
• requests bolt12 invoice by sending invoice_request via onion message
• handles invoice and invoice_error responses
• initial support for paying bolt12 invoice
  • currently 'abusing' bolt11 data structures like LnAddr, to minimize changes in payment sending code path
  • bolt12 invoice currently stored in Invoice.lightning_invoice with a prefix, because adding a new field to an @attr.s object is a PITA when switching branches, final storage location TBD.
• CLI commands add_offer, get_offer, delete_offer and list_offers for managing offers.
• handles incoming invoice_request and replies with invoice if request can be matched with an offer.
• accepts incoming HTLCs with blinding
• initial Qt GUI should be working for sending to offer
• initial QML GUI should be working for sending to offer

notes w.r.t current state:

• only first path in invoice_paths is tried, iteration of multiple path is TODO
• needs gossip enabled, no support for trampoline routing yet.
• expired offers are not automatically removed from the wallet db.
• adds extra blinding field to HTLCs in adds log, making the wallet file non-backward compatible

[ecdsa]

I have pushed a new version of https://github.com/spesmilo/electrum/commits/test_blinded_payment_onion This branch adds a unit test for the blinded payment onion creation test vector from the BOLT

[accumulator]

rebase after #10230

[ecdsa]

I pushed a commit to your branch, that adds a regtest for the scenario alice pays carol throught bob. Please try to make this work first; we can worry about the GUI later.

Edit: sorry, I had to force-push, because I made a mistake. The branch is now rebased on master.

[ecdsa]

adds extra blinding field to HTLCs in adds log, making the wallet file non-backward compatible

there should be a wallet_db upgrade for that