fides: add `PrivacyDeclaration.flexible_legal_basis_for_procesing` field

[Adam Sachs]

Is your feature request related to a specific problem?

Follow up to https://github.com/ethyca/fideslang/issues/176

Add a PrivacyDeclaration.flexible_legal_basis_for_processing field to our DB model here in fides, and ensure the field on the pydantic model (added in https://github.com/ethyca/fideslang/issues/176) is exposed properly in the application API.

Describe the solution you'd like

• update PrivacyDeclaration DB model class
• add a migration to add column to the DB table
• update any tests to ensure property can be set and read via /system API

Additional context

See https://github.com/ethyca/fideslang/issues/176 for additional context

[Roger Plotz]

moving to done