Support catalog sub-set indexing #157
Description
Currently each time the indexer executes it traverses the entire STAC catalog from the root catalog URI.
This approach is adequate for one-time or scheduled indexer runs, but may be very inefficient if no or very few items or collections are changing. In some scenarios it may be desirable for some form of event system to trigger index updates for individual entries that execute faster.
A basic implementation might support invoking the indexer with individual item or collection URIs rather than a root catalog URI, and should accommodate the potential for multiple concurrent or overlapping invocations and the risk of an ever-growing queue of invocations if the indexer is invoked with a high frequency and cannot keep up.