fix(data-warehouse): Parse the datetime for vitally sources correctly

[Gilbert09]

Problem

• The new pipeline was complaining about how vitally value conversions were erroring due to the datetime parser expecting a string but was getting a datetime instead
• Example of an error: https://posthog.sentry.io/issues/6162104915/?project=4508444747956225&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&statsPeriod=14d&stream_index=13

Changes

• Handle both cases