Use metadata.json schema as basis of our metadata types

[hobofan]

As of recently there is now a JSON Schema for the metadata.json files (bazelbuild/bazel-central-registry#1149), which we should use as the basis for the types we use in this repo.

• We should add some codegen that generates Typescript types based on the schema (possibly based on a copy of the schema in our repo, so that changes in the format upstream don't immediately break our builds)
• We should optimally add some check that will alert us that the schema in our and the upstream repo are drifting