Allow user-specification of TLS provider

[cmccoy]

#501 adds support for using OpenSSL for TLS instead the JDK (via Netty ALPN). The current PR tries OpenSSL first, falling back to JDK if OpenSSL support isn't available.

It would be helpful to have a configuration value to specify the TLS transport which should be used, raising an exception if it's unavailable (rather than falling back on a slower method).

[sduskis]

gRPC configuration was malleable enough to make this work. We ought to have this available some time today.

[sduskis]

By default, tcnative is chose if both are configured. Any code we write is going to be more confusing than test code managing the inclusion of the tcnative jar.