As part of the unified header validation component, the HTTP connection manager will be updated to honor the configured header validation policy on a per-request basis. Initial policies available will include:

• Legacy (default) - use the current, non-UHV, codec and library specific header validations
• UHV - use the Envoy maintained unified header validation component within Envoy

Eventually there will be an option available to use a custom implementation of the UHV API, instead of the Envoy maintained one.

The HCM will honor the header validation policy for each request and configure the codec/UHV appropriately. HCM-level validation is currently performed in ActiveStream::decodeHeaders.

This is blocked until #19753 is completed.