[REQ][typescript-fetch] allow tree shaking of unused API methods

Is your feature request related to a problem? Please describe.

We have a large API with hundreds of endpoints and so the typescript-fetch generator creates very large classes. As a result of the way the API is generated, if we want to use a single endpoint, we end up importing the code for all the other endpoints as well. This results in a heavy cost of downloading unused JS when visiting our website

Describe the solution you'd like

Use functions rather than classes. This would allow bundlers such as Vite, Rollup, and webpack to remove any unused code, which is something they cannot do with classes. For a discussion of why unused class methods cannot be removed, see https://github.com/rollup/rollup/issues/349#issuecomment-348406169

Describe alternatives you've considered

Use this other code generator instead. These clearly support tree shaking:

• https://github.com/Embraser01/typoas
• https://github.com/oazapfts/oazapfts

And I think this one may as well:

• https://github.com/ferdikoomen/openapi-typescript-codegen/issues/696

The issue targets the typescript-fetch generator but names no implementation files or tests. Start by locating that generator and tracing how API classes and methods are emitted; done means generated clients use functions in a way that lets Vite, Rollup, and webpack remove unused API methods.