Support salience annotation formats from open-source platforms

[g8a9]

Part of our API accepts human rationale annotations to evaluate explanations on plausibility metrics. It is reasonable to assume that human workers will annotate text with such rationales using open-source tools such as Label Studio.

In this regard, we might want to support off-the-shelf some of the output formats these platforms have. E.g:

• Label Studio: list of spans, where each span is a tuple with (starting index, ending index)