refactor(dispatcher): use generic error placeholder for the ApiClient

[mariocynicys]

Used a type Error for the ApiClient that depends on the concrete impl.

note: avoided calling request.to_endpoint_schema()? in to_data_request so that the error from to_endpoint_schema isn't exposed in the trait ApiClient and not have to comply with Self::Error in non-concrete impls.

[Alrighttt]

Hey, good work, thank you. I've created quite a few conflicts in https://github.com/KomodoPlatform/sia-rust/tree/feat/fetch-utxos-helper .

I am merging feat/fetch-utxos-helper. Please leave this open, and I will handle the conflicts. Some portions of your code are no longer needed such as the ApiClientHelpersError related code.