tapchannel: add FetchAssetMetaForAsset to AssetSyncer interface

With the address book now implementing that method, we can add it to our
AssetSyncer interface so we can query asset meta information during
channel funding.

Author: guggero

tapchannel/aux_funding_controller.go

@@ -183,6 +183,11 @@ type AssetSyncer interface {
 	// for issuance proofs.
 	QueryAssetInfo(ctx context.Context,
 		id asset.ID) (*asset.AssetGroup, error)
+
+	// FetchAssetMetaForAsset attempts to fetch an asset meta based on an
+	// asset ID.
+	FetchAssetMetaForAsset(ctx context.Context,
+		assetID asset.ID) (*proof.MetaReveal, error)
 }
 
 // FundingControllerCfg is a configuration struct that houses the necessary