MINOR: filters: Introduce filter classes and per-side class ordering

MINOR: filters: Introduce filter classes and per-side class ordering

Introduce a class registry distinct from the legacy filter keyword registry.
A class has a unique name, some flags, a instance parser and independent
request/response placement relative to other classes.

Export the built-in class names from their implementations and establish
their default order centrally. Provide registration and placement helpers
for additional classes, and reject classes placed on neither side.

Leave built-in instance parsers unset until the parser adapters are added.
This is configuration infrastructure only; stream evaluation still uses the
legacy filters. This WIP registry also unconditionally references the Lua
class: support for builds without Lua remains to be completed.