Make RestApplication class generation configurable

https://github.com/OpenAPITools/openapi-generator/pull/17646
made the generation of a RestApplication class mandatory. This causes an issue when an application has more than one OpenAPI defined endpoint.

For example, with multiple OpenAPI defined endpoints and an existing @ApplicationPath annotated class intended to group those endpoints under a common '/rest' path prefix, using openapi-generator-maven-plugin 7.3.0 or greater will fail to deploy because there will be multiple generated RestApplication class conflicting with the existing @ApplicationPath annotated class:

6-Aug-2024 21:09:58.596 SEVERE [http-nio-8080-exec-4] org.apache.openejb.observer.ObserverManager$MethodInvocation.invoke error invoking org.apache.tomee.webservices.TomeeJaxRsService@2fb68ec6
org.apache.cxf.service.factory.ServiceConstructionException
...
Caused by: org.apache.cxf.service.factory.ServiceConstructionException: There is an endpoint already running on http://localhost:8080/rest.

The mandatory generation of a RestApplication implies a single OpenAPI defined endpoint, and this solipsistic approach means to continue to use an endpoint common path prefix requires one and only one of the defined endpoints to be nominated as the 'chosen one' to define the path prefix;

servers:
  - url: /rest

This is clumsy and non-obvious.
It would be cleaner and more flexible to simply provide a configuration option to disable the RestApplication generation.

Start by tracing RestApplication generation in the openapi-generator-maven-plugin, using the behavior introduced by PR 17646 and the 7.3.0 change as context. Determine where generation is configured and how multiple OpenAPI endpoints interact with an existing @ApplicationPath class; done means a configuration option can disable generation so those endpoints deploy without conflicting RestApplication classes.