Add contentURL to distribution elements in JSON-LD

[vchendrix]

Closes ess-dive/ess-dive-catalog#415

Type of change

• [X] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Other - Built and deployed metacat image to data-test3 and confirmed that the contentUrl shows up in the json-ld view. See https://data-test3.ess-dive.lbl.gov/catalog/d1/mn/v2/views/json-ld/ess-dive-2b878a83491af99-20210830T190526171

Test Configuration

• Metacat Version: 2.14.1