Migrate SDK generator to hey-api

[StableCrimson]

There is pretty much no way to avoid a breaking change. Draft PR on personal fork while I work out the details.

hey does not have full feature parity with the openapi generator, so we can't migrate this without a breaking change. My idea is to make the generation as close to the current output as possible, and then slowly move toward the hey-style output so we can take advantage of things like tree shaking.

Key Differences

• Classes and models don't live in individual files. There is an issue open (and a few closed ones) asking about adding this.
• All class methods are static. No need to create or manage instances. There is a feature PR open that will allow for us to configure that.
• Config (such as auth key and base URL) can be stored in the generated client singleton and doesn't need to be passed around. Though this issue is open and is a feature that's on the roadmap.
• No JSDOC tags are generated. There's an issue showing interest in its implementation.