Add `serviceManager` prop to the Jupyter Context

[echarles]

As discussed with @fcollonval the JupyterContext could receive an serviceManager created from the outside instead of creating its own.

[echarles]

... or only specific aspects of the serviceManager could be provided (content, kernel(specs)...), see the current definition of a serviceManager. This would allow to create a more composable Jupyter Context with contents provided from A and kernels provided by by B.

    interface IManagers {
        /**
         * The builder for the manager.
         *
         * @deprecated will be removed in JupyterLab v5
         */
        readonly builder: Builder.IManager;
        /**
         * The contents manager for the manager.
         */
        readonly contents: Contents.IManager;
        /**
         * The events service manager.
         */
        readonly events: Event.IManager;
        /**
         * A promise that fulfills when the manager is initially ready.
         */
        readonly ready: Promise<void>;
        /**
         * The server settings of the manager.
         */
        readonly serverSettings: ServerConnection.ISettings;
        /**
         * The session manager for the manager.
         */
        readonly sessions: Session.IManager;
        /**
         * The kernel manager of the manager.
         */
        readonly kernels: Kernel.IManager;
        /**
         * The kernelspec manager for the manager.
         */
        readonly kernelspecs: KernelSpec.IManager;
        /**
         * The setting manager for the manager.
         */
        readonly settings: Setting.IManager;
        /**
         * The terminals manager for the manager.
         */
        readonly terminals: Terminal.IManager;
        /**
         * The user manager for the manager.
         */
        readonly user: User.IManager;
        /**
         * The workspace manager for the manager.
         */
        readonly workspaces: Workspace.IManager;
        /**
         * The nbconvert manager for the manager.
         */
        readonly nbconvert: NbConvert.IManager;
    }
****