Defines FRAME metadata types for Substrate runtimes.

Originally part of the substrate repository, it was extracted here as part of paritytech/substrate#8615.

Starting from v8, all historical metadata type definitions are retained. These can be enabled via features e.g. v13. The latest version is enabled by default. All legacy versions are available under the legacy feature.

The intention is to provide tooling here in the future to handle the different versions of metadata.