[agent-base] allow for https session reuse

[lukekarrys]

agent-base exports an agent that extends http.Agent. The secureEndpoint property works to make instances almost functionally equivalent to what you would get from https.Agent (defaultPort, protocol, getName, etc).

But the core https.Agent also includes SSL session reuse which would be nice to have.

Two options I can see here:

• export another class that extends https.Agent. Users can choose to use this but it kinda defeats the purpose of the rest of the work agent-base does of having a single agent.
• Use the secureEndpoint property to conditionally call https.Agent.prototype methods like getName does. Probably more work to implement and test but better UX.