With this change, java records will be serialized using their automatic constructors and accessors. Since records are not supported in JDK8, reflection is used for inspecting record metadata.

I created a new google-cloud-firestore-jdk17 module, for two purposes:

1. Test classes for records, which require jdk17
2. DocumentId, PropertyName, and ServerTimestamp annotations with @target(ElementType.RECORD_COMPONENT), which also require jdk17

Fixes #1152 ☕️

Some comments and questions:

1. Since only automatic constructors are supported, exclusion of fields is not supported
2. Custom getters are not supported
3. The existing ThrowOnExtraProperties & IgnoreExtraProperties annotations are supported
4. I put the new logic outside of CustomClassMapper for readability, and extracted some of the existing logic out of it, for reuse. Everything is package-private. It can all be easily moved (back) into CustomClassMapper.
5. I based the tests on existing tests for POJOs, and adapted them to records
6. Two of the test classes use the com.google.cloud.firestore java package, in order to have access to FirestoreImpl, which is needed for mocking
7. Should the new google-cloud-firestore-jdk17 module be moved to a separate project?
8. The code currently accepts any annotations with the simple names DocumentId, PropertyName, and ServerTimestamp, for flexibility. That can be changed to require the FQNs.