[Feature request] Accept an onion encrypted route instead of invoice for Loop In

This is a receiver's privacy related request to prevent the Loop server knowing the destination of an offchain transaction.
The receiver node could avoid exposing it's pubkey and the channel funding UTXO to the sender by providing an encrypted onion route instead of an invoice.

Loop In could post the encrypted onion route to pay after the onchain HTLC is received.

To create the encrypted onion route instead of an invoice support from LND would probably be necessary.

No files, tests, or entry points are named. Start by tracing the Loop In flow and assessing the required LND support for creating and posting an encrypted onion route; done would require an agreed design and implementation criteria for replacing the invoice.

Written by the indexing model from the issue text.