MEDIUM: filters: Parse and finalize filter-config instances

MEDIUM: filters: Parse and finalize filter-config instances

Add filter-config with a class, optional id, enabled marker and raw class
arguments. Require identifiers for classes allowing multiple instances,
reject duplicate local declarations, and allow inherited or implicit ones to
be replaced locally.

After configuration parsing, validate class multiplicity, call each class
parser once per instance and flatten the per-side class trees. Extend check,
init, per-thread and deinit callbacks to the finalized instances, without
processing instances present on both sides twice. Let FCGI lookup find
finalized instances when no legacy filter is configured.

Legacy filters and instances coexist, and the stream evaluation path is not
switched to instances by this patch. The enabled state is recorded but
runtime attachment and legacy/instance selection remain to be implemented.