sensors: Create `sensor trig` subcommand

This PR relocates the sensor trigger sample application from the
`sensor_shell` sample to a subcommand in the actual sensor shell. The
subcommand has a UI for enabling a given trigger on a given sensor.
A built-in handler for the data_ready trigger is included that prints
the latest data to the log. Currently, only `SENSOR_TRIG_DATA_READY` is
supported but the groundwork is there to add others. Tested on a
`tdk_robokit1` board.

Signed-off-by: Tristan Honscheid <honscheid@google.com>