[feature]: add RPC endpoint for generating LN Invoices for Tap assets

[ffranr]

Description

This issue proposes the implementation of a new RPC endpoint to facilitate generating a lightning network invoice for receiving tap assets over a lightning channel.

Arguments

The RPC endpoint should accept the following parameters:

• AssetId: Identifier for the asset the receiving node wishes to receive.
• Amt: Amount of the specified asset that the receiving node wishes to receive.
• RoutingPeer: A serialized, compressed Bitcoin public key (route.Vertex) identifying the routing peer. This peer acts as the final hop in the BTC payment channel, converting BTC into the AssetId and paying the invoice-generating node in the specified tap asset.

Invoice Generation Functionality

The tapd node should execute the following steps to generate an invoice:

1. Check the given AssetId. If the asset is unknown, retrieve asset details from a known universe.
2. Utilize the RFQ (Request for Quote) service to negotiate and finalize a quote with the specified routing peer. This step is crucial for determining the scid (short channel ID) for the invoice, the BTC amount to be used in the invoice, and the final expiry time of the invoice.
3. Interface with lnd to generate an invoice based on the agreed terms.
4. Amend the last hop hint in the generated invoice using the RFQ quote to specify the routing peer, ensuring correct routing and asset conversion.

The endpoint should return the modified lightning invoice to the caller.

[ffranr]

This RPC endpoint should return a status channel instead of directly providing an invoice, given that invoice generation can be time-consuming due to processes like RFQ negotiation, universe synchronization, and calls to LND. Implementing a status channel in this endpoint will prevent the caller from being blocked while these operations are executed.

[ffranr]

I don't think we've got a solid plan in place for this issue. Right now, for the PoC, this functionality is in lightning terminal.

[dstadulis]

Moving to v0.4.1 as IIUC it's related to lnd operating in remote mode