Add nullable column support

[JayWhite2357]

Background and Motivation

Currently, we have not implemented support for columns with null values. This is a very useful and ubiquitous primitive for any database engine.

Changes Required

• [ ] Support adding a flag (or some other mechanism) to the columns types in order to support nullable columns. Logically, this is the same as adding a new column datatype for each existing datatype.
• [ ] Support existing operations on nullable columns. (e.g. we should be able to add a nullable bigint to a non-nullable bigint)