Implements `json_metadata` RPC call (#692)

* Adds `impl_json_metadata!` for generating all metadata of a runtime

* Adds `json_metadata` RPC call

* Adds simple test for the `json_metadata` RPC call

* Implements json metadata in the demo runtime

* Fix indent

* Adds missing copyright headers

* Dispatch json metadata renamings and improvements

* Replaces `format!` & `String` with `Vec<JSONMetadata`

* Implements `Encode` and `Decode` for JSONMetadata

* Make `impl_json_metadata!` compileable on `no_std`

* Adapt the client to decode the correct type for `json_metadata`

* Fixes compile error and warning

* Whitespace