Support for individual fields in the entity object

New individual object on entities
- Applicable when type: "individual".
- Fields added under individual:
  - national_identification_number (object)
    - type (string; supported values: passport, driving_license, national_id_card, tax_id_number, residence_permit, other)
    - number (string)
  - date_of_birth (string; ISO 8601 date format YYYY-MM-DD)
  - nationality (string; ISO 3166-1 alpha-2 country code)
Webhook payload expansion
- Entity webhooks now include individual object in data when type: "individual".
- Events affected:
  - entity.created
  - entity.updated
  - entity.submitted
  - entity.approved

Entity Management endpoints accept and return individual fields
- Updated request/response schemas for:
  - POST /v3/entity
  - GET /v3/entity/{id}
  - PUT /v3/entity/{id}
  - POST /v3/entity/{id}/submit
- Responses include individual object when type: "individual".
Entity submission requirements for individual entities
- On full entity submission (POST /v3/entity/{id}/submit), the following are required for type: "individual":
  - individual.national_identification_number.type
  - individual.national_identification_number.number
  - individual.date_of_birth
  - individual.nationality
- For simplified entity creation/update, these fields remain optional.

individual.date_of_birth must match YYYY-MM-DD.
individual.nationality must be a 2-letter ISO country code.
individual.national_identification_number.type and .number must be provided together.

No breaking changes; additions are backward-compatible and ignorable by existing clients.