[@loopback/sequelize] Nested Loopback model types are not supported

### Describe the bug

If you have existing Loopback model properties set up that are nesting other Loopback models such as this "Address" example: https://github.com/loopbackio/loopback-next/blob/be0c53e30bbe9edf7753c4fcd7ab3199f7cd3b8c/packages/repository-json-schema/src/__tests__/unit/build-schema.unit.ts#L478-L518 it will lead to an error about unsupported types.

### Logs

```shell
Example error:

Error: Unhandled DataType "class Address extends repository_1.Model {
}" for column "address" in sequelize extension
```

### Additional information

References:
https://loopback.io/doc/en/lb4/Model.html#json-schema-inference

### Reproduction

N/A

Start by reviewing the linked repository-json-schema test covering nested Address models and the LoopBack model JSON-schema inference documentation. Then trace how the Sequelize extension handles the reported unsupported DataType error; done means nested LoopBack model properties are accepted without that error and the relevant behavior is covered by a regression test.

Written by the indexing model from the issue text.