Add RFC-0059: Starlark as programmable middleware for vMCP

Proposes extending vMCP's Starlark engine from composite-tool-only
scripting into a unified programmable middleware surface, replacing
the growing set of independent config knobs (optimizer, filter,
rate limiting, PII scrubbing) with a single script per session.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Jeremy Drouillard <jeremy@stacklok.com>

Author: jerm-dro