Add date support to StudentEducationOrganizationAssociation

Summary of outcomes for Data Standard 2.1

• Problem Addressed: An EducationOrganization's relationship with a student may change over time but the current model only allows for a static representation of the relationship.
• High-level Reasoning: Adding a BeginDate and EndDate to StudentEducationOrganizationAssociation allows the history of changes in an EducationOrganization's responsibility for a student to be preserved.
• Changes Made: Added date BeginDate to StudentEducationOrganizationAssociation as part of identity and added date EndDate as optional property

Original ticket description follows:

Recommendation: Track BeginDate and EndDate at the entity level - to track the start and end of a District’s official responsibility for a student.