Add a declaration.Constants class

The declaration.Constants class will regroup all the keys employed to configure
the Declarations (ImportDeclarations & ExportDeclarations)
Add fuchsia-helpers modules

fuchsia-helpers module contains a FuchsiaHelper class, and an abstract class
to facilitate tests on ImporterService implementations.
Add JSON-RPC ImporterService implementation

The JSON-RPC ImporterService use the jasborb implementation of JSON-RPC
Add JSON-RPC ImporterService integration tests
Fix the DefaultImportationLinkerTest in core-it

Add a missing mandatory ServiceProperty in the instance creation