Allow for an ingestion of more than one item

Currently the API only accepts single items https://github.com/developmentseed/pgstac-ingestor/blob/38a2a55c7bd255684383550844b8e40323b97f69/pgstac_ingestor/main.py#L32.

For scenarios where one wants to ingest in one shot a large number of items in bulk (say, 500k items stored in an ndjson at once), but where a direct connection to the DB (and therefore simple usage of `pypgstac`) is unacceptable, it's quite inefficient to use the API to insert one by one all the items.

It would be desirable to expose an API for one-shot large ingestion tasks.

Thoughts @sharkinsspatial @vincentsarago ?

Start with the existing single-item API in pgstac_ingestor/main.py at line 32. The issue proposes a bulk-ingestion API for large sets of items, such as an NDJSON file; clarify the API design and expected behavior before deciding what would count as done.

Written by the indexing model from the issue text.