AMP Support for SendPaymentRequest in lnd.py, for lnd v0.18 and above.

[lft3t8bx]

AMP Support for SendPaymentRequest in lnd.py, for lnd v0.18 and above.

When users submit an amp invoice, the payout would fail if the coordinator is running lnd v0.18 and above with the following error at the lnd backed:

[lncli] rpc error: code = Unknown desc = the AMP flag (--amp or SendPaymentRequest.Amp) must be set to pay an AMP invoice

This PR simply adds two linse in lnd.py (under pay_invoice and under follow_send_payment SendPaymentRequest) to enable AMP support when sending payments, as shown below:

request = router_pb2.SendPaymentRequest(
    payment_request=lnpayment.invoice,
    fee_limit_sat=fee_limit_sat,
    timeout_seconds=timeout_seconds,
    amp=True  # Added this line
)

This PR has been tested by Temple Of Sats coordinator for around 1 week.
No issues have been detected so far.

Checklist before merging

• Install pre-commit and initialize it: pip install pre-commit, then pre-commit install. Pre-commit installs git hooks that automatically check the codebase. If pre-commit fails when you commit your changes, please fix the problems it points out.

[KoalaSat]

Merging to be included on the next release. Thanks @lft3t8bx for testing this out