Implement Disable() on handler configurers.

[jmalloc]

What change does this introduce?

This PR adds an IsDisabled() method to the Handler and RichHandler interfaces, the return value of which is driven by calls to the Disable() method on Dogma's handler configurer interfaces (AggregateConfigurer, etc).

Why make this change?

I wanted to prototype this before merging dogmatiq/dogma#162

Is there anything you are unsure about?

I haven't looked into adding support for Disable() to the static analysis system. I also haven't added support to the interopspec config API.

What issues does this relate to?

• Add Disable() method to configurers. dogma#162

[jmalloc]

/cc @danilvpetrov FYI this will likely need some modifications to the static analysis code.

[jmalloc]

Thinking some more about the static analysis side of things, I'm not sure we can even report the disabled status in any meaningful way, because c.Disable() would always be called conditionally (otherwise the handler would always do nothing).

Edit: I guess we could handle the case where c.Disable() is called unconditionally, but otherwise just report the handler as enabled.

[jmalloc]

After discussion with @danilvpetrov, we decided to change the IsDisabled() method to instead return an enum of Enabled, Disabled and ConditionallyDisabled allowing us to represent all three possibilities from static analysis.

When the config is constructed from an in-memory application the return value will always be either Enabled or Disabled.

[jmalloc]

I've updated this PR to exclude any handlers that are disabled and otherwise unconfigured (i.e. no identity or routes) from application configurations.