feat: implement is_negative_hits in rate limit descriptor

[rudrakhp]

Summary

Add support for the is_negative_hits field on rate limit descriptors. When set, the
hits_addend value decrements the rate limit counter instead of incrementing it, effectively
refilling previously consumed tokens. The counter is floored at 0 (cannot go negative).

• Upgrade go-control-plane/envoy to v1.37.1 (adds IsNegativeHits proto field)
• Redis: uses an atomic Lua script (EVAL) to decrement and floor at 0 in a single operation
• Memcached: uses native Decrement which floors at 0 by default
• Negative hits bypass over-limit and near-limit checks (always return OK)
• New total_negative_hits stat tracks requested decrements per descriptor

Fixes envoyproxy/envoy#41219