[feature]: create asset aware HTLC interceptor RPC

[guggero]

As long as lightningnetwork/lnd#7162 isn't addressed, there can only be one interceptor for HTLC forwards.
And that single slot is occupied if tapd is connected to lnd.

Therefore, someone operating litd with Taproot Asset channels isn't able to use the HTLC forward interceptor to add custom application logic to their node.

Proposed solution

Add a bidirectional streaming RPC to tapd with a structure similar to the HtlcInterceptor RPC in lnd that forwards events from lnd.
The RPC should allow developers to:

• Reject or allow HTLCs (of any type, including non-asset/BTC-only HTLCs).
• See all asset-related metadata tapd uses to make asset routing decisions. For example:
  • The RFQ policy associated with an asset HTLC.
  • The decision/response the RFQ order handler would return to lnd.

Open questions

• Should the RPC user be able to override a "deny" decision from the RFQ handler?
• Should the RPC user be allowed to modify the decision/response of the RFQ order handler?

[ZZiigguurraatt]

Add a bidirectional streaming RPC to tapd with a structure similar to the HtlcInterceptor RPC in lnd that forwards events from lnd.

So this then requires a tapd macaroon and a potentially a different TCP socket to access? Not sure if I think this is the cleanest solution because what if a user of the node has no idea that tapd is being used by the node and that they need to try and use the tapd interceptor instead of the lnd interceptor?