use json schema for hive.json

We can declare a JSON schema for the hive.json file and also expose it via HTTP so that people can reference it via the [`$schema` property](https://json-schema.org/understanding-json-schema/reference/schema#dollarschema), for better IDE support.

CLI docs: https://the-guild.dev/graphql/hive/docs/api-reference/cli
CLI commands that reference `hive.json` keys: https://github.com/kamilkisiela/graphql-hive/blob/5bef13b15e05b639c76ea7d847045d017a12c8a7/packages/libraries/cli/src/commands
Example config file: https://github.com/kamilkisiela/graphql-hive/blob/7fafb0774fab3feefbf40c06855d8a9041b0e55a/packages/libraries/cli/hive.json