Rename apstra_managed_device to apstra_agent

[chrismarget-j]

The managed_device resource was named after the "managed devices" tab in the web UI.

That tab leads to a management page where two different resources are managed in concert:

• Agents via /api/system-agents
• Systems via /api/systems

The first attempt (in this resource) attempts to manage both together, but it's now clear we're mostly managing the Agent, and we need to be able to refer to them individually via terraform resources and data sources.

The apstra_managed_device resource should be renamed apstra_agent, and the underlying structs should reflect that change as well.