feat(host): fetcher builder to customize hint types & implementations

[zobront]

Is your feature request related to a problem? Please describe.
Different users may have slightly different needs in terms of what data is saved by the fetcher. Currently, the only choice is to fork Kona and modify it. It would be helpful for users to be able to override hint implementations or even add new hint types when importing zkvm-host as a library.

Describe the solution you'd like
An interface similar to the accelerated precompiles where new hint types can be registered or overridden when building the fetcher.

Describe alternatives you've considered
N/A

Additional context
N/A