Gather uniqueness constraints from databases for records schema

[vinceatbluelabs]

We should populate the 'unique' variable in fields in records schemas based on whether there's a uniqueness constraint set in the database.

https://stackoverflow.com/questions/14355499/how-to-model-a-unique-constraint-in-sqlalchemy

https://docs.sqlalchemy.org/en/13/core/constraints.html

https://www.vertica.com/docs/9.2.x/HTML/Content/Authoring/AdministratorsGuide/Constraints/ConstraintTypes/UniqueConstraints.htm

https://docs.aws.amazon.com/redshift/latest/dg/t_Defining_constraints.html (note that there's no enforcement by redshift)