Add JSON schema for metadata

[HCastano]

This PR introduces a JSON schema which describes the contract
metadata. There are two schemas, the outer metadata (generated by cargo-contract) and
the versioned ink! metadata.

(This distinction only exists because cargo-contract is technically agnostic to the
language metadata, although in practice it's really only using the ink! metadata. In the
future we should be able to merge these into a single schema.)

The schemas provide a source of truth about what key-value pairs are supported and what
types they expect. This is useful for developers as well as external tooling. One use
case here is tooling which needs to verify that a given piece of metadata is valid.

Note that the generated process for the schemas is currently using some hacked branches
across scale-info, ink, and cargo-contract (the generation process is described in
the metadata crate's README). My goal is that in the future we'll be able to properly
upstream these changes across our repos and have an easy way to generate the schemas from
cargo-contract. At that point we'll be able to get rid of these hardcoded schema files.

Related issue: #940

[cmichi]

For some whatever reason I cannot add suggestions to the readme. I'm approving it hence now and might create a follow-up PR.

For the Metadata Schema Generation, do you plan on upstreaming those changes?

[HCastano]

For the Metadata Schema Generation, do you plan on upstreaming those changes?

Yeah. I think the ideal workflow would be to have an option in cargo-contract to generate the schema. Will take a bit to upstream though since it cuts through a few repos

[ascjones]

LGTM

[HCastano]

CI failures are unrelated to this PR