fix: handling of varied SharePoint date formats (#2591)
We are seeing occurrences of inconsistency in the timestamps returned by
office365.sharepoint when fetching created and modified dates.
Furthermore, in future versions of this library, a datetime object will
be returned rather than a string.
## Changes
- This adds logic to guarantee SharePoint dates will be properly
formatted as ISO, regardless of the format provided by the sharepoint
library.
- Bumps timestamp format output to include timezone offset (as we do
with others)
## Testing
Unit test added to validate this datetime handling across various
formats.
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Co-authored-by: David Potter <potterdavidm@gmail.com>
