Add possibility to check values to index validator

[markusbaden]

Sometimes we want to validate that a DataFrame contains certain columns, without necessarily worrying about what is the content of that column.

In my case I am parsing a file as part of an ETL process and want to check the result. My expectation is that the file will always have the same columns in the same place. Say the columns I am expecting are ['a', 'b'], then

pd.DataFrame(
    [
        [1, 2], 
        [3, 4],
    ],
    columns=['a', 'b'],
)

would be valid, while both

pd.DataFrame(
    [
        [2, 1], 
        [4, 3],
    ],
    columns=['b', 'a'],
)

and

pd.DataFrame(
    [
        [1, 2], 
        [3, 4],
    ],
    columns=['x', 'y'],
)

would not be valid. This case is very strict (i.e. order of columns matters) and we might want to relax this in future iterations.