signal handling: allow Envoy Logger to be ostream for signal handling

Currently Event::SignalAction installs std::cerr as the only ostream passed during signal handling on termination. For Envoy Mobile it would be far more useful to allow Envoy Logger to act as ostream so crash scopes could be output to platform-specific crash APIs.

Start by locating Event::SignalAction and tracing how std::cerr is installed as the termination-time ostream. Check how Envoy Logger could be exposed at that boundary for Envoy Mobile's platform-specific crash APIs. Done means crash scopes can be emitted through Envoy Logger during signal handling without losing the existing termination behavior.

Written by the indexing model from the issue text.