Adding signed ingestion

[bgmello]

This PR adds support for a new signed URL file upload workflow as an alternative to the existing direct ingest endpoint.

This is an optional flag that speeds up uploads and makes them more reliable, specifically for Aleph Pro. But OpenAleph may choose to implement this as well.

Instead of a single multipart POST to /collections/{id}/ingest, the new flow works in three steps:

• request a signed upload URL;
• PUT the file content to that URL;
• than create the document record via /collections/{id}/document.

This is opt-in via a --signed-url flag on the CLI crawldir command, and a signed_url=True parameter on the AlephAPI.ingest_upload() Python method.

The old ingest path remains the default so nothing breaks for existing users.

New unit and integration tests were added and all pass.