filemanager: add way to shorten attribute API querying

[mmalenic]

As described in #462, ideally there would be a way to shorten querying top-level attributes:

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

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

This is a bit of a challenge, as described in #492, because it involves capturing unknown parameters in the API.

This may not be the best idea, because it could lead to confusion and conflicts between the known parameters and unknown parameters. E.g. if the user typed ?object_id=... it might be better to reject this request rather than assuming that this request was for the attributes. Note, that currently unknown parameters are not rejected with an error status code, which should probably be changed.

Some ideas on how to implement this:

• Allow any unknown parameters and assume that the user intends to query by attributes.
• Add a list of "allowed" parameters which can be used to query attributes. For example, just allowing portal_run_id and other important attributes. These could be configurable during deployment.

[reisingerf]

A shorter / dedicated way to query for attributes would be nice, yes.

I'd have thought something like this though:
/api/v1/s3_objects/attribute?portal_run_id=...

That should allow the differentiation between parameters of the s3_object itself and any additional parameters.

[mmalenic]

Nice, yeah, I think that's a good solution.