Goal:

Add an option for building Envoy that uses OpenSSL for encryption.

Acceptance Criteria:

Building with the appropriate options/flags produces an Envoy build that uses OpenSSL for all encryption related features.

Open questions:

Presumably, the implementation will collect patches to the Envoy code using `#IFDEF` adding envoy-openssl as a dependency, where envoy-openssl is the BoringSSL/OpenSSL compatibility layer. If that is the case, should envoy-openssl be versioned to support specific version combinations of Envoy (BoringSSL) and OpenSSL, e.g. Envoy 1.32 and OpenSSL 3.0, 3.2, 3.5.

Depending on specific features of OpenSSL, other functionality within Envoy may need to be disabled. For example, QUIC support in OpenSSL < 3.4.