[feature]: Support MPP record (alias Payment_Address) for more RPC calls

[ziggie1984]

We should always enforce MPP payments because they include the payment_secret which has been introduced a while ago and therefore we should also be able to include the MPP record in the following RPC-Calls.

They are now required in the specification for writer and reader of an invoice, and when sending via a route we should also make sure we include an MPP record.

relates to #9752

Tasks to do:

1. QueryRoutes should support including an MPP record (payment_addr) when querying the route which can then be used in SendToRoute
2. SendToRoute should also enforce an MPP record.

Extras:

3. Allow AMP Record in buildroute
4. Allow Setting AMP record for QueryRoutes
5. Allow SendToRoute with an AMP record

[PriyanshusSGupta]

I want to contribute

[ziggie1984]

This should also include the buildroute rpc: #9752 (comment)

[kwsantiago]

@ziggie1984 I've been working on an initial implementation for this, pushed up what I have so far to #10091. There's a lot of work left but at least it's a starting point. Would love feedback/suggestions.

[malsomesh9]

Hi! I see this issue has been stale for a while. I'm a Summer of Bitcoin applicant and I'd like to pick this up and finish the work started by the previous contributor. Is it okay if I take this over?

[lightninglabs-deploy]

Consolidating related issue here. The following has been closed as a duplicate:

• Querying and sending payment APIs should accept total amount and payment address #4020 — Original request (from lnd 0.9.0 era) that QueryRoutes/SendToRoute and other payment APIs should accept total_amount and payment_address fields. Notes a workaround: SendToRoute can accept the payment address, but you must manually mutate the route, and QueryRoutes won't properly account for the TLV record size (workaround: add dummy custom records when querying, remove when sending).