Support authentication using CLI and WeaverClient

[fmigneault]

Description

Some useful authentication handlers such as https://github.com/Ouranosinc/requests-magpie can be provided to auth parameter of requests to automatically resolve the authentication method before submitting a request.

Weaver should provide a way to support this kind of handlers when targeting a server that requires authentication.

For Python WeaverClient, this is straightforward. Simply expose an auth parameter that is passed down to any request operation it accomplishes. The user will then be able to call the client with its own implementation.

For the CLI, allowing custom auth handler implementations is a slightly trickier.
Parameters should be defined in a way to allow runtime instantiation of the auth handler implementation.
For example, https://github.com/Ouranosinc/requests-magpie, would be referenced as such:

weaver <operation> --auth-impl "requests_magpie.MagpieAuth" --auth-args "<magpie-url>,<magpie-user>,<magpie-pwd>"

When detecting those arguments, the CLI should instantiate the auth handler accordingly, and then pass it around as in the case of WeaverClient.