Allow overriding of process method in BaseApiListingResource

[splatch]

Currently it is not an easy task to customize Swagger lookups so they work in other way than through servlet context. This causes troubles for example for CXF which is using single servlet to deploy multiple endpoints. It will also allow to invalidate cached swagger instance in certain cases, when classes are changed (this is usecase in OSGi).

Currently lookups of Swagger in context are hard to customize and would require bigger API changes.