Support `format: uuid`

### Motivation

Our API spec heavily uses UUID-typed fields, which are represented in our OpenAPI doc using `format: uuid` as described in the [JSONSchema spec](https://json-schema.org/understanding-json-schema/reference/string#resource-identifiers).

However, `swift-openapi-generator` currently ignores that part of the OpenAPI doc and generates them all as `Swift.String`, meaning we have to manually call `UUID.uuidString` and `UUID(uuidString:)` (and implement required error handling on the latter).

### Proposed solution

Make the generator respect `format: uuid` and define such fields using the Swift `UUID` type.

### Alternatives considered

_No response_

### Additional information

_No response_

No file or test is named in the issue. Start by tracing the generator's OpenAPI schema type-mapping entry point for string formats, then verify that fields declared with format: uuid generate as Swift.UUID rather than Swift.String and that the existing generated-code tests pass.

Written by the indexing model from the issue text.