[feature]: if only a single group_key or ungrouped asset_id present in the wallet, use that by default #1540
Description
Throughout all RPC I think that if tapd's wallet only has a single group_key or ungrouped asset_id present in the wallet, use that by default when a group_key or asset_id is expected. It's likely most users are only going to have a single asset, USDt, in their wallet. We should make onboarding and usage of taproot assets as simple as possible for them by not confusing them with multiple assets if they don't actually have multiple assets.
If there are multiple group_key or ungrouped asset_id present in the wallet, I think we should keep the current behavior and require one of them to be passed to the RPC.