Add Now() method to all scope interfaces.

[jmalloc]

The current time is something you need to obtain often in the handler. We've hit a real requirement for this when writing a ProjectionMessageHandler.Compact() implementation that needs to remove data that is older than some threshold.

While in general I do not want to make the engine responsible for providing every possible dependency to the handlers, the fact that dogmatiq/testkit lets you scale/advance time artificially means that we do need some way to query the engine for the time it "thinks" it is.

Edit: I'm not sure we neccessarily want to add it to all of these interfaces, but I have put as asterisk next to those that I think probably need it most.

• AggregateCommandScope
• ProcessEventScope
• ProcessTimeoutScope
• IntegrationCommandScope
• ProjectionEventScope
• ProjectionCompactScope

[jmalloc]

After discussing with @danilvpetrov and @koden-km we have decided only to implement ProjectionCompactScope.Now() until we have some concrete use cases for the others.

[jmalloc]

This issue has come up again in a concrete use case at cwx. TL;DR, an aggregate implementation is using time.Now() and it is therefore hard to assert about an exact timestamp in testkit. I think we should go ahead with adding Now() to all scopes.