Export Value and ParameterValue type aliases

[jsurany]

resolves #78

Describe your changes
Value type has been replaced by ColumnValue and ParameterValue types, both of which are exposed via both cdb2.py and dbapi2.py modules. Updated anything that uses the Value type to now use one of those two types.

Testing performed
Currently no tests. I could create a series of tests that look like below, but I'm not sure how value-added that is, since none of the other exports from any modules are similarly tested.

# test_exports.py
def test_column_value_is_exported_from_dbapi2() -> None:
    from comdb2.dbapi2 import ColumnValue

or I could build a test that creates a fake package that imports these types and then runs mypy, but again I'm not sure how value-added that is.

Additional context
Spoke with Matt about this change and he suggested breaking it into the two types, since Value was used for both parameters and for column values returned inside result sets. I've adjusted functions that use Value to use one of the two replacements. We aren't going to similarly expose the Row type since it just aliases Any, and it's not a big DevX burden to just use Any for Row.

[godlygeek]

We aren't going to similarly expose the Row type since it just aliases Any, and it's not a big DevX burden to just use Any for Row.

And more importantly, because exposing Row as an alias for Any would be a commitment that might get in the way of eventually making the Handle and Connection and Cursor types generics that are parametrized by row type.

Spoke with Matt about this change and he suggested breaking it into the two types, since Value was used for both parameters and for column values returned inside result sets.

I'm gonna go back on that a little bit. I'm going to suggest we leave a Value type for backwards compatibility, but use it only for the column values, and add a new ParameterValue that's used for values that can be passed as parameters. I'm gonna make that change.

[godlygeek]

After my changes, this LGTM, but I'd appreciate another set of eyes since I changed it pretty heavily.

@sarahmonod Mind giving this a review as well?

[sarahmonod]

LGTM too!