Add Disable() method to configurers.

[jmalloc]

What change does this introduce?

This PR introduces an Disable() method to each of the handler configured interfaces.

Why make this change?

It is often necessary to disable specific handlers by adding conditional logic to the application's Configure() method. For example, during tests we will often skip registering a projection handler if there is no SQL database available for testing.

This change allows the handler itself to decide if it should be disabled, rather than the application.

Is there anything you are unsure about?

I've added an as-yet-unused DisableOption in the interest for forward-compatibility. I'm hypothesising options such as a DisableIf(predicate func() bool), which could act as a circuit breaker that stops an engine from routing messages to a handler while some dependency is unavailable.

What issues does this relate to?

None

[danilvpetrov]

Makes sense to me to introduce these methods.