MINOR: filters: Add helpers to iterate and find filter configurations

MINOR: filters: Add helpers to iterate and find filter configurations

Add flt_foreach_conf() to walk the filter configurations of a proxy in
the evaluation order of a given side, and flt_find_conf() to look up the
first configuration matching an id and an optional callback.

Both helpers select the legacy filter configurations (px->filter_configs)
when the proxy is in legacy mode, and the finalized filter instances
otherwise, following the global, request or response order. They will let
the filter callbacks and the action parsers work in both modes without
duplicating the list selection.

The helpers are not used yet.