MINOR: flt-decomp: Use the filter helpers for implicit decompression checks

MINOR: flt-decomp: Use the filter helpers for implicit decompression checks

Rework check_implicit_decomp_flt() to walk the filter configurations with
flt_foreach_conf() in global order and keep the request/response detection,
the cache ordering and the explicit declaration detection in a dedicated
callback.

This makes the implicit request and response decompression filters work
with the filter instances, where the legacy filter_configs list is empty.
In instance mode the implicit declaration rules are enforced by the
instance parsers, so the legacy-specific checks are skipped.

The locally declared implicit filter configurations preserve the previous
behavior and are inserted at the beginning of the legacy list.