[feature]: Make vPSBT creation available via the APIs

Background

While it is currently possible to create PSBT swaps with Taproot Assets, this is not easily doable as not all the necessary API calls are available.

Reading through the PSBT swap itest gives an outline of the process and many of the steps are available via the API, key generation, PSBT signing, etc. What is missing is the creation step, an API call for the creation of the vPSBT.

It looks as if the vPSBT creation requires the ForInteractiveSend() function which is not currently exposed via an API.

Is your feature request related to a problem? Please describe.

This issue was discovered when attempting to experiment with PSBT swaps. Walking through the itest example I was unable to find a way to create a vPSBT via API calls.

Describe the solution you'd like

Developers looking to experiment with this feature would benefit greatly from this functionality being exposed via an API call.

Describe alternatives you've considered

Once I realized that the API call for creating vPSBTs was not available I looked at a number of different methods for doing this. I could export the ForInteractiveSend() function and run it in my code base to create the PSBT there, however, it seems that this function relies on other functions, and access to the DB would be needed.

The only other method I found for making this happen is to fork the repo and add custom functionality.

Start with itest/psbt_test.go around the PSBT swap flow and inspect ForInteractiveSend(), which the issue identifies as the missing API capability. Trace the existing API calls for key generation and PSBT signing, then define the API path that exposes vPSBT creation and verify the complete flow can be performed through API calls.