It would be nice to be able to extend a reservation if needed

Example: I reserve a machine for 48 hours... at 36 hours I realize I need it for another 48 hours to finish my work. It would be nice to be able to tell testflinger to extend my reservation (I have the reservation job id, right?) ... perhaps:

testflinger-cli extend-reservation --timeout=

and the agent would extend the timeout by

Start with the testflinger-cli entry point for reservation commands and trace how the reservation job ID reaches the agent timeout. Define the behavior for extending an active reservation by the requested time, then verify that the CLI and agent agree on the resulting timeout.

Written by the indexing model from the issue text.