refactor: move methods implemented in Client to corresponding Backend methods

[datapythonista]

xref #2678

Follow up of discussion in #3014

Seems like it's quite challenging at this point to make Backend.connect() methods return an instance of `Backend. See this comment for a list of the possible options we've got.

Regardless of implementing this particular change (making connect return Backend instead of Client), we've been working on moving methods from Client classes to Backend classes, and then creating a proxy method in Client that calls the method in the new location. This was done while standardizing the API.

A first step towards the new backend API structure would be to finish moving all the remaining methods. This will need to eventually be done. And even if we don't fully remove Client, or if the API is not fully specified or standard, it'll greatly simplify the code of backends having all methods in the same classes, and not arbitrarily split between Client and Backend.

To move forward with this approach, the next PRs will be open:

• Delegate all objects not found in Client to the corresponding Backend instance
• Move methods from Client classes into Backend, one backend at a time

[cpcloud]

Partially addressed by #3127.

[cpcloud]

@datapythonista Does #3127 fully address this issue?