CC-45: Added support for multiple timestamp columns

[mikkin]

With this PR, users can specify multiple timestamp columns to address the scenario where they may have "modified" and "created" timestamp columns. They will need to list the column names in the order of priority. For e.g. modified, created - will look at modified column first and only if that column is null, it will proceed to created

There are checks in place to ensure that at least one of the timestamp column is not null.

Finally, added tests to validate the changes made.

[mikkin]

@wicknicks, @kkonstantine, @rhauch please review

[wicknicks]

Thanks for the PR, Mikkin. LGTM.

[mikeakala]

Is there a plan to release this feature?

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