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

Nobody has claimed this yet.

`FetchAssetMeta` currently requires one of the following

https://github.com/lightninglabs/taproot-assets/blob/e621beefce9d80a5fed091b52534876c23db5e15/taprpc/taprootassets.proto#L1301-L1317

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, https://github.com/lightninglabs/taproot-assets/issues/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.

Start with the FetchAssetMeta definition in taprpc/taprootassets.proto at lines 1301–1317, then review the related discussion in issue #1503. Clarify whether group_key support belongs in this RPC or a new one, and define how the metadata array and separate decimal_display value should be returned for every asset in the group.

Written by the indexing model from the issue text.