ASIO: add PaAsio_GetSampleRate() function

[Spacechild1]

PaAsio_GetSampleRate() returns the current sample rate of the specified device at the time of library initialization.
(If the current sample rate is unknown, a sample rate of 0 will be returned.)

For example, this is useful for people who want to use the current sample rate as the default sample rate (which PA's ASIO implementation does not do.)

Actually, I would rather prefer the solution proposed in #970, but if this is not desired, this PR would be an acceptable workaround.

However, the function would still be useful for other purposes, e.g. to select the "current" sample rate - instead of the "default" sample rate - in a UI dialog.