Third-party SDK: consume JS-declared extensions from other OSGi bundles

[romain-pm]

Part of EPIC #554. Implemented — under review in #687.

New exported package org.jahia.modules.javascript.modules.engine.sdk with JSServerExtensionInvoker: forEach(type, handler) iterates JS registry entries within one GraalVM context, Invoker.call(...) executes a JS callable and converts results to plain Java — consumer bundles need no polyglot or engine-internal imports (those packages stay unexported). The engine bundle's Export-Package lists only this package.

First consumer: formidable (JS form-field validators executed server-side).