Enhancement: Custom FFmpeg Path Support for FFMPEG Backend

[HemantKArya]

This pull request enhances the audioread library by introducing custom FFmpeg path support for the FFMPEG backend. With this enhancement, users can specify a custom path to the FFmpeg executable, enabling greater flexibility and compatibility across different systems.

Key Changes:

Added functionality to specify a custom path to the FFmpeg executable for the FFMPEG backend.
Improved compatibility by handling platform-specific differences in FFmpeg executable paths (e.g., .exe extension on Windows).

Example use with change:
with audioread.audio_open(os.path.realpath(path), backends=backends,ffmpeg_path=ffmpeg_path) as input_file: sr_native = input_file.samplerate n_channels = input_file.channels

We can ignore the ffmpeg_path argument if doesn't have custom ffmpeg path, cause it will detect it from environment variables PATH.
So now it is compatible with both with ffmpeg_path and without it both at the same time.