Manage relationships bundle-collection-product in API

[tloubrieu-jpl]

The relationships that need to be managed are:
bundle --> collections : for a given bundle, all collections can be retrieved
collection --> products: for a given collection, all observational products can be retrieved.

Acceptance criteria:
Check that the relationship management is:

• documented in the online documentation : https://nasa-pds.github.io/pds-api/
• is implemented in the reference server (https://github.com/NASA-PDS/registry-api-service) deployed in demo on https://pds-gamma.jpl.nasa.gov/api/swagger-ui.html