Add metadata description RPC

[apopiak]

The runtime metadata is currently only returned as a SCALE encoded blob that needs external descriptions to be decoded.
Let's add an RPC endpoint that returns some useful description of the metadata (even if it is created manually short to medium term).

related to #917

[bkchr]

What is useful description of the Metadata?

[shawntabrizi]

What is useful description of the Metadata?

Probably a JSON blob in whatever format Polkadot JS needs to connect to the node successfully.

Imagine a JSON file which is compiled into the Wasm and returned when calling this RPC endpoint.

To keep your Polkadot JS UI running, you simply keep this JSON up to date as you upgrade your Wasm.

A manual and temporary solution, but should work.

[bkchr]

You mean the types.json?

[shawntabrizi]

Yup. We basically want to bake it into the node, or at least give the option to. At least until the real solution is complete to support types in the metadata by autogeneration.

Just one idea.

[bkchr]

We could introduce some new metadata version that adds support for adding arbitrary data?

[xlc]

We could introduce some new metadata version that adds support for adding arbitrary data?

#3686