C#: Attr.cs: Mark a few functions as public

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
C#: Trigger.cs: Modify get_rate() / set_rate() to use Attr.read/write

This is made because the functions iio_trigger_get_rate /
iio_trigger_set_rate are no longer available.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>