Add functions to parse CLI-level format of priorities

The current serialisation format for submit priorities is
integer-based, same as the opcode json serialisation. But for CLI
level, we need to support a string-based format, so we add functions
to parse and format this representation.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>