invoice+rpc: add exit hop InvoiceAcceptor sub-systems and RPC calls

[ffranr]

Change Description

Closes https://github.com/lightningnetwork/lnd/issues/8616

This pull request introduces the following components:

1. An interceptor service for invoice settlements.
2. An invoice acceptor RPC server and endpoint which makes use of the invoice settlement interceptor.
3. An integration test demonstrating the settlement of an invoice through the invoice acceptor, using a HTLC with an amount smaller than the specified invoice amount.

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[dstadulis]

@jharveyb and @GeorgeTsagk will review

[ffranr]

I think I also need to pass circuit key for latest HTLC along with invoice to settlement interceptor client. This way the client can inspect the correct HTLC.

[ffranr]

I think I also need to pass circuit key for latest HTLC along with invoice to settlement interceptor client. This way the client can inspect the correct HTLC.

This is included in the latest commits.

[ffranr]

I'm in the process of writing an itest for this PR. I noticed that providing the exit HTLC circuit key to the acceptor interceptor client is insufficient. The invoice will only include the exit HTLC if it is a replayed HTLC.

I will therefore need to provide fields from the invoice update context to the interceptor client. I will make changes to that effect.

[GeorgeTsagk]

reviewed diff, left out the WIP commits also haven't locally tested anything

[ffranr]

Latest state:

1. New integration test which shows that an invoice can be forced into the settled state (using the new interceptor) when the exit HTLC amount is less than the invoice amount.

2. The latest modifications account for the MPP invoice settlement flow.

3. I've simplified and reduced the necessary code changes.

Things to consider:

1. The smallest exit HTLC payment amount I could send was 1000 msat. I'm not sure if we're hopping to send 0.

2. We may need to add additional fields to the interceptor client request (provide more data to the interceptor client). In particular the custom records that are sent along with the p2p messages (UpdateAddHtlc etc.). That is not currently implemented.

[jharveyb]

1. The smallest exit HTLC payment amount I could send was 1000 msat. I'm not sure if we're hopping to send 0.

IIRC the plan was for the HTLC to be >= dust threshold / 330ish sats, so that seems fine.

[lightninglabs-deploy]

@roasbeef: review reminder @jharveyb: review reminder @georgetsagk: review reminder

[ffranr]

I need to double check whether there are modifications in the PoC branch which relate to this PR.

[guggero]

Replaced by https://github.com/lightningnetwork/lnd/pull/8771.