Persist xpub/Derivation Paths used by Minter during PSBT minting

[dstadulis]

Context:

https://github.com/lightninglabs/taproot-assets/tree/cold-group-key
While assessing the MVP functionality of PSBT-based minting (to enable users to mint assets with signatures produced by sending a PSBT to a signing device s.a. a hardware signer) the commit 74f5a32 enhances Minter functionality to produce correct PSBT with proper derivation info.

Remaining functionality necessary for production-ready PSBT minting:

• Persist xpub/derivation paths used by Minter

The goal is to ensure, if tapd is terminated during the minting process, that the paths / xpubs are stored to disk / DB

Non-MVP enhancement

• Enable resumption of minting from persisted xpub/derivation paths