Add unknown type (2.x)

[WoH]

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Closing issues

Closes #452

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• Has the issue had a maintainer respond to the issue and clarify that the feature is something that aligns with the goals and philosophy of the project?

Potential Problems With The Approach

unknown and any are treated equally at the moment.
This should not be an issue since the main difference should be how ts handles type checking, which tsoa doesn't care about.
However, this means there's no new validation logic (same as any) or dataType etc.

Compatibility

New, backwards compatible feature, so I'm targeting master.
Will be ported for 3.x aswell.