feat: DTOSS-8752 - initial validators for NBSS Appointment event files

[ianfnelson]

Description

Added initial validators for NBSS Appointment event files

Context

Begins adding validator classes to ensure quality of incoming NBSS data.
More to come in future PRs.

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.