This pull request introduces support for external workspace management, allowing users to register and manage workspaces that are provisioned and managed outside of the Coder.

• coder external-workspaces create - Creates a new external workspace (this command extends coder create)
  • Example: coder external-workspaces create ext-workspace --template=externally-managed-workspace -y
  • Checks if template has coder_external_agent resource before creating a workspace
• coder external-workspaces list - Lists all external workspaces
• coder external-workspaces agent-instructions - Retrieves agent connection instruction
  • Example: coder external-workspaces agent-instructions ext-workspace main --output=json