Allow additional schema options on Operation.parameter/5

[mrnovalles]

When trying to add additional fields to the OpenApiSpex.Schema sent as an option to the Operation.parameter function, these options suddenly dissapeared and all that was returned in the resulting JSON was the :type key.

This commit adds an additional step to check if the :schema key was passed as an option before falling back to building a schema solely including the parameter type. This will be useful when displaying for example enum: ["foo", "bar"] for a query param taking only some values

Fixes #563