Add rudimentary XPCSession support

[reesericci]

This pull request adds basic XPCSession support for the parts of the library reliant on ExtensionFoundation. However, I found out the hard way that ExtensionKit still doesn't support XPCSession, so most of the library is untouched.

All of the new XPCSession support is prefixed with "XPC" as to not break backwards-compatibility.

Related to #2

[mattmassicotte]

Thanks for looking at this! We will have to be a little more careful to make sure the public API is not changed at all, as extension systems are particularly sensitive to breakage in that way.

However, I don't quite yet understand how this implementation adds XPCSession support. Can you help me understand the plan a little better?

[reesericci]

It adds an XPCConnectableExtension into the library, which wraps around ExtensionFoundation in the same vain as the current ConnectableExtension and requires a ConnectionHandler. Most of this is untouched because ExtensionKit wrappers, the vast majority of codebase, doesn't support XPCSession.

[mattmassicotte]

Ok, yes. What I do not understand yet is how that change helps move towards the goal. In fact, I see you've removed the init I added to try to match the XPCSession stuff that was added.

Don't get me wrong, I'm very interested in doing this. I just don't fully see the plan you have for how to get there yet. I'm sorry I'm being so slow...

[reesericci]

It kinda doesn't. It just allows for passing/creating XPCSession based handlers to the parts of the library that don't rely on ExtensionKit (which doesn't support XPCSession).