[GOBBLIN-1912] Highwtm query record parsing fix

[gautamguptabasant]

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-1912

Description

• We updated the Salesforce high watermark query to use MAX aggregate function instead of order by with limit 1, as part of [GOBBLIN-1888] Optimizing high watermark metadata query for SFDC #3750
• The response for the aggregate query is a bit different from the previous version.
• Updating the parsing logic accordingly as part of this PR.

The new response looks as follows when there is no valid result of the MAX query:

{
    "totalSize": 1,
    "done": true,
    "records": [
        {
            "attributes": {
                "type": "AggregateResult"
            },
            "expr0": null
        }
    ]
}

When there is a valid value, it looks as follows:

{
    "totalSize": 1,
    "done": true,
    "records": [
        {
            "attributes": {
                "type": "AggregateResult"
            },
            "expr0": "2023-09-15T05:21:41.000+0000"
        }
    ]
}

Tests

• Added UTs to test parsing of the response in various cases.

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"