[feature] - tapdb+universe+rpc: expose outpoint-based universe-leaf queries over RPC

[Roasbeef]

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

Today users have two ways to query for a Universe leaf:

• QueryProof: allows to query based on a combination of: universe ID, script key, and also outpoint
• AssetLeaves: queries for the entire set of leaves for for a given ID

With just the behavior of the existing calls, there isn't an easy way to look up any asset related proofs given a txid/outpoint. Lack of such an API makes it harder to build a simpler overlay explorer (eg: click/hover an outpoint for extra info) for taproot assets.

Describe the solution you'd like

Resolve this TODO:

taproot-assets/rpcserver.go

Lines 5037 to 5041 in b154a99

	default:
	// TODO(roasbeef): can actually allow not to be, then would
	// fetch all for the given outpoint
	return leafKey, fmt.Errorf("script key must be set")
	}

Additional context

The existing query partially supports this behavior: https://github.com/lightninglabs/taproot-assets/blob/b154a998c8b1215310cdfb3e4f897a56cb4ee587/tapdb/sqlc/queries/universe.sql#L56C10-L71

It needs to be made less strict, also allowing the namespace value to be unspecified.

[dstadulis]

This could be used in an integration for mempool.space etc

[dstadulis]

Existing product with similar queries:
https://megalithic.me/taproot_assets_explorer?search=b8ef8e9030445fac119944190bf2f7a3026c9a12090fe8e422227577707d9ae1%3A0