Add validation for RequestItem#orig_number

Validate it using a generic method for checking North American numbers.
Add validations for RequestItem#term_number

Extract generic optional phone number expectations to shared examples
Add validations for RequestItem#bill_to_number

- DRY up tests
- DRY up validation methods
Replace regex with slightly more efficient Array#include?
Fix up Request validations
- Validate required and optional fields
- Rename validation methods to match business terms
- Refactor Request validations for clarity