[GOBBLIN-2040] Abstract comparable watermark

[ZihanLi58]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-2040

Description

• Here are some details about my PR, including screenshots (if applicable):
  This PR is to abstract comparable watermark so that it support different type of offset value.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  Unit test and test in Azkaban to make sure this is a backward compatible change

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[priyah16]

I don't see any tests that validates this change. How is it verified? Could you help confirm?

[ZihanLi58]

There is no API change in this class at all. i.e. the getValue still return String type and the way to construct the class still remain the same. So I think we don't need test for this part. Let me know if you still have concerns about this.