Support readOnly/writeOnly properties next to a $ref

[bijancn]

Note that this is not compatible with OpenAPI 3.0.3 which states that the $ref should replace everything. However, in OpenAPI 3.1.0 will be compatible to JSON schema which allows in 2019-09 to add any keywords next to a $ref. In older versions JSON schema also didn't have this but they figured out that it's more intuitive like this rather than using allOf as a workaround. We already used this syntax for adding defaults, so we will continue to be slightly ahead of OpenAPI (that is until 3.1.0 is released which is currently in RC0).

[Stefanqn]

since we're using the openAPI parser internally, it would be fine for me to use RC0 in this case