gptel-transient (gptel-send-menu): Add new transient-prefix
`gptel-send-menu' for setting API options for the buffer
and (optionally) sending queries. Various infix options are supported.
2023-03-09: Some transient menus are generated dynamically, which
requires a more recent version of transient than the latest tagged
release. As a result, the dynamic generation code is commented out for
now.
