refactor!: use FutureOr to allow synchronous interceptors

[Leptopoda]

This allows synchronous implementations for interceptors.

[CodingAleCR]

Interesting, what is your use case?

Also, it would be great if you update the examples and the docs along with the library's code.

Thank you!

[Leptopoda]

Sorry for my delay.
The idea is that we mostly use synchronous interceptors so one can add a separate subclass that only allows synchronous methods.
If used in a custom http client one would not get the async suspension.