Support DATE and DATETIME in MSSQLToGCSOperator

[QuinRiva]

closes: #11370

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JSON serialization thows errors if date/datetime fields are in the table. Converting those fields to strings before serialization solves this issue.

Additionally add the DATETIME (which is separate to timestamp) as a typemap for converting a field to bigquery. This has the unfortunate side-effect of converting DATE to DATETIME as the MSSQL DBAPI doesn't distinguish between DATE and DATETIME (both type 2).

[boring-cyborg]

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[ashb]

Please add unit tests.

[ashb]

Please add unit tests.

[kaxil]

Can you please rebase your PR on latest Master since we have applied Black and PyUpgrade on Master.

It will help if your squash your commits into single commit first so that there are less conflicts.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[potiuk]

Is this something we should finish/merge @ashb/@kaxil ? it looks needed :)

[mik-laj]

@potiuk This is about integration with Google, so looks like we'll have to take care of it.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.