MEDIUM: flt-decomp: implement decomp-req and decomp-res filters

MEDIUM: flt-decomp: implement decomp-req and decomp-res filters

Initial filter implementation for decompressing gzip/zlib streams by
leveraging libslz/uslz API.

For now both request and response filters only work for TCP proxies
(with "mode tcp" set). While this is currently not possible to
compress TCP streams using the existing compression filters, thus
making the TCP decompression less useful than the upcoming HTTP
decompression, basic tcp decompression implementation helped during
development to stress test the libslz decompression part without
falling in the haproxy HTX API pitfalls straight away. However it
turns out the TCP decompression may find some use-cases, so let's
keep that for now.

With TCP decompression now supported, we have all the code
infrastructure needed to perform HTTP decompression, which will come
in a later patch.