feat: Add guest execution agent and vsock support

- Introduced a standalone guest execution agent to facilitate command execution within the guest VM over Firecracker's vsock.
- Enhanced FirecrackerClient to support vsock configuration, validating guest CID and UDS path.
- Updated configuration to include vsock settings by default.
- Refactored FirecrackerProcessManager to manage runtime sockets more effectively.
- Implemented guest agent installation and provisioning in the image store.
- Added API endpoints for executing commands in the VM and installing the guest agent.
- Improved error handling for guest execution failures and agent availability.

uvm/errors.py

@@ -39,6 +39,18 @@ class FirecrackerError(UvmError):
     """Raised when Firecracker cannot be started or configured."""
 
 
+class GuestAgentUnavailableError(UvmError):
+    """The VM's execution agent could not be reached before submission."""
+
+
+class GuestExecutionError(UvmError):
+    """Guest execution failed; a lost response does not prove the command failed."""
+
+    def __init__(self, message: str, *, outcome_unknown: bool = False) -> None:
+        super().__init__(message)
+        self.outcome_unknown = outcome_unknown
+
+
 class TapCreationError(UvmError):
     """Raised when TAP setup fails after creating a host interface."""