Stream Filter Support

[tonurics]

Please create a Stream Filter context on zstd.* that can be used with the stream_filter_append and stream_filter_prepend Stream Functions.

This would bring the Zstd extension in parity with the current Zlib extension.

Reference: zlib_filter.c

Related Tickets:

• Support incremental compression and decompression #64
• Is an "ob_zstdhandler" function possible? #71