Related to #1067

• Modifies the existing POST /workspaces
  • it exclusively handles workspace creation — the ability to rename via this endpoint is removed
  • it can handle custom instructions & muxes configuration as part of the same request
• Adds a PUT /workspaces/:workspace_name
  • this is how you rename a workspace now
  • this can also handle updating custom instructions & muxes configuration

Has a companion PR for the dashboard to handle the breaking changes: stacklok/codegate-ui#349

ToDo:

• GET /workspaces/:workspace_name or maybe (/workspaces/:workspace_name/config, I'm not sure yet) to get the configuration for a workspace