Mutual Authentication for JDK Providers

Motivation:
Netty's SslContext classes currently do not support mutual authentication. It would be nice to have these interfaces support mutual authentication and then initially provide an implementation for the JDK server/client providers.

Modifications:
-Add extra constructor methods in the SslContext classes to support taking trust store and key store related items in both the client and server code
-Add a test case which uses mutual authentication to verify functionality

Result:
Netty's SslContext interfaces support mutual authentication and the JDK client/server provides implement this interface.