Commit 8974c38 doesn't provide backward compatibility

[MathewNWSH]

Describe the bug
Commit 8974c38 which introduced assets attributes indexing, doesn't provide backward compatibility.

To Reproduce
After running newer sfeos app on older os db:

  File "/tmp/stac-fastapi-elasticsearch-opensearch/stac_fastapi/core/stac_fastapi/core/serializers.py", line 185, in db_to_stac
    a.pop("es_key"): a for a in collection.get("assets", [])
    ^^^^^
AttributeError: 'str' object has no attribute 'pop'

Expected behavior
Provided updates should work with existing instances, also indexing of assets attributes could be optional.

[Zaczero]

In our case, we don't really need to index assets, so ideally we would want to disable this feature altogether. Having a config switch would make it both backwards-compatible and more customizable for the user.

[jonhealy1]

Making this functionality an option makes sense I think.

[jamesfisher-geo]

Agreed.

cc. @rhysrevans3

[rhysrevans3]

Sorry about that, I'll add a flag to make the functionality optional and a script for updating existing indexes to work with the changes.