Implement Elide JSON API service in pass-core

[markpatton]

• [x] Change path to /data
• [x] Discussed pluralization and decided not to support it for now
• [x] Add back in methods from original Java model and inherit from PassEntity
• [x] Put all Converters in own package
• [x] Put back methods that were stripped out
• [x] Examine attribute types and add Serdes as appropriate
• [x] Must fix code formatting problems
• [x] Maven should build the Docker image
• [x] Copyright statements in code files
• [x] Enable github ci actions
• [x] Docker compose setup should use postgres

[markpatton]

Initial version of pass-core with the Elide JSON API service committed: https://github.com/eclipse-pass/pass-core

[jabrah]

• No strong opinion, but I'd lean towards /data. Do we want or need to have API versioning? For example, the default JSON:API namespace from Elide was /api/v1
• +1 for pluralization to follow convention. From the UI perspective, singular version that we have now needs a small customization to work in Ember, but it's a 1-liner, so not a big deal