[feature]: Persist External Key for Seedlings During Minting Process

[ffranr]

When minting using an external key, the minter can assign an external key to each seedling in a batch. The external key is a struct composed of three fields: xpub, master_fingerprint, and derivation_path.

Currently, during the minting process, the MintAsset RPC endpoint is used to add a seedling to the batch and save it to disk. However, the external key associated with the seedling is not being saved to disk. This means the external key does not persist if the process is interrupted or restarted.

To improve the user experience, ensure that the seedling's assigned external key is persisted to the database using the InsertAssetSeedlingIntoBatch function and can be retrieved when needed.

By persisting the external key, the external key will survive a tapd restart between minting steps: MintAsset and FundBatch.