External (federated) content should be indexed to allow it to be queried

The federation framework should provide a way for external content to be indexed and queried. One option is for connectors to use events to signal new content. Another option might be to specify whether content should be indexed when a new projection is created.

There should also be a way for connectors (and even other JCR clients) to use a ModeShape-provided mixin that signals that the content should not be indexed.

For example, a connector might have some content that is expensive to iterate over content (e.g., a Git connector might prevent/discourage iterating over all commits in the history), yet allow navigation (e.g., navigate to a specific commit in the history).