Introduce support for operations outside of message handler

[SeanFeldman]

Closes #82

With this change, messages can be sent/published outside of the message handler, similar to what can be done with IEndpointInstance or IMessageSession. This enables scenarios such as dispatching messages from Timer and HTTP triggers, using both the static and FunctionsHostBuilder approach.

Using FunctionsHostBuilder approach with HttpTrigger:

Dispatching a message on TimerTrigger-ed Function:

[boblangley]

Should IFunctionEndpoint now also extend IMessageSession ?

[SeanFeldman]

Should IFunctionEndpoint now also extend IMessageSession ?

That's how I initially started, with extending IFunctionEndpoint with IMessageSession.
The downside of that approach was that IMessageSession is not designed for Functions runtime and ExecutionContext and ILogger are not part of the signature. Therefore Send and Publish w/o those two would be possible but not desirable. I'm also not convinced that having IMessageSession would add much value as DI would provide IFunctionsEndpoint. And it's consistent across the Functions implementation with a smaller public API surface.

For these reasons, I think trying to introduce IMessageSession is not needed.