Manage field preselection in queries

[tloubrieu-jpl]

As a user, I want to be able to select the fields I want in my search response of collections.

The management of null fields should be handled:

• when no fields are selected, all available fields are returned (no null fields)
• when fields are selected, all selected fields are returned. When they are not available they are set to a "null" value. This value should not be mis-interpreted for an original "null" in the pds model (to be detailed).

Acceptance criteria: Check that specification is clear about the syntax to select fields in the JSON response, see:

1. https://github.com/NASA-PDS/pds-api/blob/master/docs/spec/pds-api-specification.md
2. and online https://nasa-pds.github.io/pds-api/
3. validate also in reference implementation https://github.com/NASA-PDS/registry-api-service deployed on https://pds-gamma.jpl.nasa.gov/api/swagger-ui.html