Make all client properties optional

[lukebakken]

Fixes #56

Originally found by @tomtaoran

[the-mikedavis]

Another option here could be to mark ClientCapabilities as #[serde(default)] and add Default to the #[derive(..)] list. That should behave the same but then any code that handles this doesn't need to mess with the Option wrappers. It's a bit cleaner if the lack of basic_nack for example is the same thing as explicitly setting false.

The same trick could be applied to ClientProperties above

[michaelklishin]

I'll do what @the-mikedavis has suggested, using Serde defaults here is arguably the right thing to do.

@lukebakken @the-mikedavis I have invited you to be collaborators in this repo.