resource(switch_port_settings): register the resource

[sudomateo]

Registered the existing oxide_switch_port_settings resource and added public facing documentation and a changelog entry.

[sudomateo]

For context we merged #426 without registering the oxide_switch_port_settings resource with Terraform and opened this pull request to re-register the resource. We'll use this draft pull request to:

• Give the networking team a place to build and test the Terraform provider with the oxide_switch_port_settings resource.
• Iterate on the design of the networking API and oxide_switch_port_settings resource.

We will merge this pull request once this resource becomes ready for customer usage or if we get the networking APIs to a point where we consider them stable for the near to mid future.