Allow to pass custom resolvers

[fabricedesre]

The main change is to switch iroh-gateway/src/client.rs#Client to be: pub struct Client<T: ContentLoader> and to sprinkle the generic and other needed bounds where needed.

iroh-one/src/content_loader.rs showcases a custom content loader. For now it's similar to the regular one without the gateway racing implementation.

[fabricedesre]

This is to fix https://github.com/n0-computer/beetle/issues/194

[fabricedesre]

note: all tests are passing locally on a Mac

[fabricedesre]

Rebased to current tree. ping @dignifiedquire & @Arqu

[Arqu]

@fabricedesre just noticed the branch is out of date. Can you rebase and we can merge.

[fabricedesre]

@fabricedesre just noticed the branch is out of date. Can you rebase and we can merge.

done!