[ENH] allow object dtype-s in pandas based Table mtype-s (#5651)

This PR relaxes the `pandas` based `Table` mtype-s to also allow object
`dtype`-s.

The reason for this is to allow programmatic support of type checking of
`y` in time series classification and regression, and since the current
implicitly defined interface supported object (e.g., string) `dtype`-s
for classification outputs.

Also required for the multioutput extension in
#5408.