fix filter queryables definition to at least have a content type and object

[philvarner]

Related Issue(s): #156

Proposed Changes:

1. fix queryables definition in Filter to at least have a content type and object. In the future, we should have a precise definition for JSON Schema, but this is quite a bit of work to write.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.