pcm: rate: add rewindable and forwardable callbacks

pcm: rate: add rewindable and forwardable callbacks

This commit does not fix nonsense values returned by the rewind and
forward callbacks. E.g., with period_size = 1024 and buffer_size = 4096,
an attempt to rewind 1024 samples from the nearly-full buffer returns
4090.

Due to these nonsense values, the current rate plugin should be treated
as non-rewindable. That's why the new callbacks return 0.

Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>