Add support for communicating BigDecimal scale/precision using multipleOf

I'm producing a JSON schema for a service that needs to communicate the precision of BigDecimal numbers.
This allows client generators to pick the right data type (i.e. not pick a float but a decimal data type).

The `multipleOf` keyword is the right one for this, e.g. https://swagger.io/docs/specification/data-models/data-types/

This is the kind of API spec I want to produce
```
type: number
multipleOf: 0.01
```

I have not found any annotation that I can use right now to do this. Maybe making this similar to [Bean Validation Constraints](https://github.com/ePages-de/restdocs-api-spec?tab=readme-ov-file#documenting-bean-validation-constraints) would be an option.