refactor(filemanager): misc simpler and clearer API

[mmalenic]

Some more misc changed, related to #462

Changes

• JSON patch now supports append only, so "add", "test", or "copy" only if an existing key is not being overwritten.
• The outer "attributes" on patch requests is not required.
• date has been renamed to eventTime for more clarity. /cc @williamputraintan

I'm holding off on implementing the shortened attributes query string feature, e.g. portal_run_id=... rather than attributes[portal_run_id]=... as described in #462. This is slightly more complex as it involves capturing unknown parameters in the API. I'm not sure if this is the best idea, because it has the potential to conflict with existing parameters.

[victorskl]

LGTM.

Noted well about the challenge with the query parameter impl. We can brainstorm together more.

[brainstorm]

Makes sense to not drop attributes[] prefix, I suspected it would clash with some other namespace and it seems it indeed does? :-/