Allow using a custom API client in rvk_methods

[awaitlink]

Summary

Allow using a custom API client in rvk_methods instead of just rvk::APIClient.

Suggested solution

This will likely involve creating a trait that has the necessary function, call_method, and implementing that trait for rvk::APIClient.

Avoiding rvk_methods depending on rvk

A possible solution for this may be to add a new crate like rvk_traits for this purpose (and also move the Params type alias there, although it might be unnecessary and should rather be entirely removed).

Additionally, rvk_methods::supported_api_client would need to be removed.

Both rvk_methods and rvk would depend on rvk_traits (the former—to use it, the latter—to implement the trait).

Unresolved questions

• Is the rvk_traits name good enough or should a better one be used (e.g. rvk_core)?
• Name of the trait.