resampler: Add assert to set_input/output_rate

This adds asserts to check if the implementation has an update rate
function defined.
resampler: Move the method strings into the implementation struct
resampler: Don't check for unsupported methods in fix_method

With split of the resample_methods table the parse_method function
doesn't allow choosing an unsupported resampling method.

This check is now dead code and is therefore removed.
resampler: Add the auto resampling implementation

This adds back the 'auto' resampling method to the supported resampler
methods.