Allow to configure proxy on a provider basis

[FWest98]

While it is possible to set the appropriate environment variables to get Go to use a proxy (as fixed in #57 ), it would be nice to explicitly support setting proxy settings in the provider configuration; since I only want the rancher connection to use the proxy and not my other connections.

Similar requests for another provider: hashicorp/terraform-provider-kubernetes#1133. It is up to the individual providers to implement this: hashicorp/terraform#17386 (comment).

[sboschman]

@a-blender , is getting a pr merged into 'norman' something you can assist with in any way? 🙏

[ericpromislow]

PR #1237 has been in draft since Sept. 2023

That PR is waiting on rancher/norman#453 which has been double-approved and
looks ok, but I don't have authority to merge norman PRs

[MKlimuszka]

Putting this on hold until we decide which release has the QA capacity to cover it.