Support for a `timeout` option in the config

[pattrickrice]

Describe the solution you'd like Hi! First off, the library is awesome!

I'd generally like the ability to specify a default timeout for the generated client similar to what axios provides as mentioned in https://github.com/ferdikoomen/openapi-typescript-codegen/issues/942. IMO I like that this library uses fetch not axios, so it would be nice to make this available without the additional dependency.