LND v0.18.3: blinded paths for BOLT 11

[kaloudis]

LND v0.18.3-beta introduces the ability to create BOLT 11 invoices with blinded paths and pay them. Let's investigate if it's ready to implement, and if so, implement for our three LND interfaces.

More on blinded paths here: https://lightningprivacy.com/en/blinded-trampoline

[kaloudis]

Related LND PRs: https://github.com/lightningnetwork/lnd/labels/blinded%20paths

[kaloudis]

Relevant proto changes:

Invoice (AddInvoice req): https://github.com/ellemouton/lnd/blob/master/lnrpc/lightning.proto#L3846

[kaloudis]

Other considerations:

• Do we need our JS bolt11 decoder to detect these flags first? (https://github.com/bitcoinjs/bolt11/)
• How do we gracefully inform users of backends that don't support bolt11 blinded paths that they cannot pay the invoice?