lightning: missing bolt 04 payment_metadata field support

[f321x]

Description

BOLT-04 specifies a payment_metadata field the sender is supposed to carry over from the bolt 11 invoice into the payment onions to allow stateless invoice issuance.
If a recipient relies on this to settle a payment i assume we couldn't pay them as we don't implement this.

https://github.com/lightning/bolts/blob/master/04-onion-routing.md

...
if the recipient provided payment_metadata:

    MUST include payment_metadata with every HTLC
    MUST not apply any limits to the size of payment_metadata except the limits implied by the fixed onion size
...