Properties that start with numbers, e.g. "5qi" generate invalid Java identifiers

[jphorx]

Any properties that begin with numbers result in generated Java code that will not compile. If generated code is changed to fix the compiler error, i.e. rename field to _5qi, the JSON converter generated from the @DataObject annotation processor omits the "5qi" property.

There are many incidences like this in published API specifications from 3GPP, particularly the dozens of 5G mobile core services. Modifying the specifications themselves is not a feasible option.

Steps to reproduce:

1. Go to https://vertx-starter.jetdrone.xyz/#maven
2. Select OpenAPI server
3. Upload test.yaml (in test.yaml.zip )
4. Generate the project (the attached test.zip) - the generated POJO code is not valid.