Add special case to check values of str column

[cosmicBboy]

Describe the bug

FYI @jeffzi @dineshkumar-23

The bug is clearly described here. Basically, since str dtype arrays are translated to a numpy object arrays, any object can exist within such a column and still pass validation.

There is now pandas.StringDtype since pandas > 1.0, but I think it's still important to special-case this type because (i) many users may not be aware of it and (ii) I think pandera should start getting into the business of correcting some of pandas' quirks, esp. when it comes to the type system.

The special-casing should be implemented at the DataType definition (i.e. pandera.engines.numpy_engine.String) after we have an API for logical data types #798.

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Code Sample, a copy-pastable example

See #807

Expected behavior

Failure cases of non-string objects in a numpy object array (aka a string column) should be correctly reported.