fix: separating create customer from customer response schemas (#142)

### TL;DR

Refactored customer schema definitions to use a consistent approach for
both creation and update operations.

### What changed?

- Created new schema definitions for customer creation requests:
  - `CustomerCreateRequest` as a base schema
- `IndividualCustomerCreateRequest` and `BusinessCustomerCreateRequest`
extending the base schema
- `CustomerCreateRequestOneOf` to handle the oneOf relationship between
customer types
- Replaced the separate update schemas (`IndividualCustomerUpdate` and
`BusinessCustomerUpdate`) with the same creation request schemas
- Updated API endpoints to reference the new schema definitions:
  - Changed POST `/customers` to use `CustomerCreateRequestOneOf`
- Changed PATCH `/customers/{customerId}` to use
`CustomerCreateRequestOneOf`

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Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: pengying