Add Zipped STAC export + Direct Download Signed URL

[notthatbreezy]

Problem description

The S3 prefix location is nice, but it is only useful if you have direct access to the S3 bucket of the export. Now that we have relative links, having a single download URL makes things easier for pulling down the STAC export and doing something cool with it.

Expected Behavior or Output

• In addition to what the STAC export job does now, it also zips up the results and puts it at the root of the s3 prefix as catalog.zip
• The list/detail views of the STAC exports in the API include signed URLs for the zip files of the catalog