Adding a trait to provide current time through the client config

[emarteca]

There are various places in the source code currently where MlsTime::now() is called. This calls the SystemTime::now (unless we are running in WASM).

This PR adds the ability to specialize the clock. This is useful both for testing (setting a deterministic clock), and also for allowing an application to provide an external clock.

[mulmarta]

Hey @emarteca ! I'm a bit hesitant to add yet more trait - something we explicitly want to avoid in the 1.x branch... Going through the code, now() is not called in too many places. Maybe using the builder pattern plus explicit inputs we can make the API clean?

• tests -- calling this in tests seems wrong, it should be some constant?
• commit -- we can have optional custom time in CommitBuilder
• validate key package -- we can have input
• key package lifetime -- custom time can be input for now, I think 1.x already has KeyPackageBuilder

[emarteca]

Thanks for the review -- and please excuse the delay in response! That makes sense. I will refactor and ping back with changes soon.

[emarteca]

Thanks again for your review -- I've refactored the code as you suggested, in #282