Expose the Jersey injection manager

[dmetzler]

Expose the Jersey injection manager to be able to use it outside of Jersey resources

Description of changes:

This PR just add a method on the JerseyLambdaContainerHandler that exposes the Jersey injections manager. It's very useful as it allows to use it to build the whole dependency tree.

// List of application bindings to configure injections, this
// allows to have different injection in tests
protected abstract List<Binder> binders();

public AbstractServiceHandler() {
  ResourceConfig jerseyApplication = new ResourceConfig()
                                             .packages("com.acme.jaxrs.resources");

  binders().forEach(jerseyApplication::register);

  handler = JerseyLambdaContainerHandler.getAwsProxyHandler(jerseyApplication);
  im = handler.getInjectionManager();
  ServiceFactory.setInstance(im.getInstance(Service.class));
}

If the lambda code also handles other type of event, the same injector can then be reused:

public String handleS3Request(S3Event s3Event) {
  Service service = im.getInstance(Service.class);
  return service.handleEvent(adapt(s3Event));
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[sapessi]

Nice, I'll merge with the next minor release

[sapessi]

Merged manually in the core branch where I'm working on the new changes. Closing this PR.