Generate mappings for APIs

# Overview

Currently, the generator plugin only downloads the root discovery, and then downloads every discovery that is included in the root discovery.

## Tasks

- [ ] Work out an ideal API structure (example: [the Google API Java Client Services library](https://github.com/googleapis/google-api-java-client-services/))
- [ ] Use a library that auto-generates Kotlin code (e.g. [Square's KotlinPoet](https://square.github.io/kotlinpoet/), which can be used with [Google's Kotlin Symbol Processing (KSP)](https://kotlinlang.org/docs/ksp-overview.html))
- [ ] Allow for devs to specify how the files should be created, and their contents

## For consideration

- Compile-time checking for OAuth scopes
- Something like an opt-in annotation might suffice, but how exactly do we surface the required OAuth scopes that an API might need?