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

[ffranr]

We have a default derivation path currently and customizing/storing isn't a useful feature (not in v0.6 with certainty?).
Please feel free to re-open if I've missed anything.
I'm going to take the initiative and close this.

[ffranr]

We have a default derivation path currently and customizing/storing isn't a useful feature (not in v0.6 with certainty?). Please feel free to re-open if I've missed anything. I'm going to take the initiative and close this.

I was slightly off here. The derivation path is customisable. But more generally, so are the other fields in the external key struct. So we need to persist the entire external key specifier.

I've replaced this issue with #1319