filemanager: consider making the attribute API more user-friendly

[mmalenic]

The attribute API uses square brackets for access, which is less ideal because they have to be percent-encoded in the URL. It might be better allow top-level JSON fields to just be accessed directly from the API, instead of nesting them.

E.g, instead of:
/api/v1/s3_objects?attributes[portal_run_id]=...

It could be:
/api/v1/s3_objects?portal_run_id=...

Similarly, the JSON patch request does not need to have the outer attributes field, as it's clear that only attributes can be patched.

E.g. instead of:

{
    "attributes": [ { "op": "add", "path": "/attribute_id", "value": "attribute_id" } ]
}

It could just be:

[ { "op": "add", "path": "/attribute_id", "value": "attribute_id" } ]