Produce a JSONSchema for the mock HTTP config.yml files

The goal is to have a JSON Schema definition for the configuration files that describe the mock HTTP endpoints. This will provide hints and validation in IDEs. It would be used like this

```
# yaml-language-server: $schema=https://server.com/elastic-stream.schema.json
---
rules:
- path: /api/v1/Foo
methods: ["POST"
```

or the IDE could be configured based on file path patterns to apply the schema, such that no `yaml-language-server` hint is necessary.

----

### Requirements

- The JSON Schema should be generated from the structs in the code to ensure the schema is always in sync with the code.
- CI should validate that the schema file in the repo is matches the code.

-----

I have had good experience using https://github.com/invopop/jsonschema to generate schemas from structs that are annotated with `jsonschema` struct tags. So I would try this path first.