SNI-routing without TLS termination: Make secretRef optional in sslConfig

[brandonkal]

I would like to route some domains without termination and terminate others as normal on the same IP. Because Envoy can do this--see closed envoy issue--Gloo should be able to do this as well.

Based on the Slack discussion, it appears that allowing secretRef to be nil should work. That approach is quite intuitive--leave out the secret and gloo's Envoy proxy will not perform TLS termination and instead route based on the tls ClientHello.
According to the envoy issue, leaving out tls_context in Envoy config should enable this to function.

[EItanya]

This was taken care of during the failover feature work