MINOR: filters: Add per-proxy filter instance storage and lookup

MINOR: filters: Add per-proxy filter instance storage and lookup

Prepare a instance model separate from the instantiated legacy filter
configurations. Keep raw arguments and source locations, and anchor each
instance in the class references for the sides on which its class exists.
Per-side flat lists will hold the finalized order later.

Create class references when setting up non-internal proxies, following the
global class order. Add instance traversal and lookup helpers, plus cleanup
for both the class-reference trees and flat lists on proxy destruction.
Prepare per-side links in flt_conf for the later runtime integration.

No configuration directive creates instances yet, and stream attachment
remains unchanged.