MINOR: flt-http-comp: Use the filter helpers for implicit compression checks

MINOR: flt-http-comp: Use the filter helpers for implicit compression checks

Rework check_implicit_http_comp_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 compression 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 appended at the end of the legacy list.