Secure Envoy profile

This issue continues from https://github.com/envoyproxy/envoy/issues/5348 and tracks the security specific concerns in a "secure Envoy" build. Ideas that we have heard about so far include:

• Conservative defaults for all buffer sizes and timeouts in the configuration.
• A SECURITY_ASSERT macro that might promote some extant ASSERT to RELEASE_ASSERT.
• Additional data structure and data plane payload integrity checks.
• Restricting allowed extensions to those tagged as valid for untrusted downstream/upstreams (see https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/security/threat_model#core-and-extensions)
• Build with Scudo allocator (https://github.com/envoyproxy/envoy/issues/9365)
• -fstack-protector-all
• -fstack-clash-protection
• https://clang.llvm.org/docs/ControlFlowIntegrity.html
• Anything else in https://wiki.debian.org/Hardening
• Enabling ABSL_HARDENING_ASSERT

Please propose others.