Expand REST API to provide annotator input/output information

[jbaker-dstl]

As of Baleen 2.4, the required inputs and produced outputs of each annotator are declared in order to the pipeline orderers to function. It would be beneficial if this information could be exposed through the REST API.

The gotcha to this is that this information is only accessible once the annotator has been instantiated and configured, so it would either need to be per annotator in an existing pipeline, or allow for annotator configuration to be passed.