Add support for pre-shared keys

[thomdev]

Client and server should have the option to use out-of-band pre-shared keys rather than certificates.

https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#Pre-shared-keys

[normanmaurer]

Yes! I will add support for it.

[normanmaurer]

As it turns out this is only for TLS1.2 and so not what we need... currently investigating how to add support for TLS1.3 which is used for QUIC.

[normanmaurer]

We may need to wait for BoringSSL to add support /cc @davidben