[feature]: `FetchAssetMeta` should accept `group_key` as an input alternative to `asset_id`

[ZZiigguurraatt]

FetchAssetMeta currently requires one of the following

taproot-assets/taprpc/taprootassets.proto

Lines 1301 to 1317 in e621bee

	message FetchAssetMetaRequest {
	oneof asset {
	// The asset ID of the asset to fetch the meta for.
	bytes asset_id = 1;
	// The 32-byte meta hash of the asset meta.
	bytes meta_hash = 2;
	// The hex encoded asset ID of the asset to fetch the meta for.
	string asset_id_str = 3;
	// The hex encoded meta hash of the asset meta.
	string meta_hash_str = 4;
	}
	}

as in input.

We should also accept group_key as there can be many asset_id in a group. Different each asset_id in a group can have different metadata, so we should return an array of metadata for all asset_id in the group_key.

However, there is a special case of data that is normally embedded in the metadata, the decimal_display. This is not supposed to change, so we should decode and return that value in a separate property. See also, #1503 . Alternatively, if we don't want to include the group's decimal_display value in the response to FetchAssetMeta, we should create a new RPC that does.