channeldb+invoices: add spontaneous AMP receiving + sending via SendToRoute

[cfromknecht]

This PR adds the ability to receive spontaneous AMP payments, and send them by crafting the appropriate request using SendToRoute. This PR is opened as a Draft since not all feature bits/TLV record types are decided.

PLEASE DO NOT RUN THIS ON A PRODUCTION NODE

Summary of changes:

• Adds support for parsing AMP record from TLV onion payload
• Adds (non-final) AMP feature bits
• Add InvoiceRef support for querying by pay_addr only
• Add JIT AMP-invoice insertion when receiving AMP HTLC
• Validation + testing around HTLC-level preimage persistence
• Reconstruction of HTLCSet child preimage/hashes using amp package
• Integration test demonstrating multi-path AMP via SendToRoute

TODO:

• expand itest assertions
• add more unit testing around AMP invoice settlement w/ invalid HTLC sets
• fix invoice parsing for AMP invoices