[Do Not Merge] Validate the registered apps existence

[tzolov]

• Add isMetadataResourceExists(Resource app) method to the App configuration metadata resolver APIs - later verifies that the metadata artifact being a metadata jar, application jar or docker image exists physically.

• Extend the AppRegistryController #register and #registerAll methods with an optional, artefact-validation boolean flag. For backward compatibility the artefact-validation defaults to false.

• The AppRegistryController#register() method with artefact-validation=true will fail with NonExistingApplicationArtefactException if the URIs are pointing to non-existing artefacts. The application registration is aborted.

• The new NonExistingApplicationArtefactException exception is added to the RestControllerAdvice

• The AppRegistryController#register() method with artefact-validation=true will store all valid registration but will remove those that faile to resovle their metadata URIs. The error message lists the failing app's URIs.

• REST client's AppRegistryOperations#{register,registerimportFromResource,registerAll} methods are extended with an optional artefactValidation parameter.

• Add some initial tests.

Resolves #1497

[markpollack]

let's speak to christian and see if this is still relevant.

[onobc]

I am closing this one, it has been in the same limbo for quite some time.