Fix #4047 Redesign notebook model to handle single client session (#4371)

* Start single client session based on the default kernel or saved kernel in NB.

* Added kernel displayName to standardKernel.
Modified name to allign wtih Juptyer Kernel.name.
So we can show the displayName during startup and use the name to start the session.

* Change session.OnSessionReady event in KernelDropDown

* Added model.KernelChnaged for switching kernel in the same provider

* Fixed session.Ready sequence

* Fixed merge issues

* Solve merged issue

* Fixed wrong kernel name in saved NB

* Added new event in Model to notify kernel change.
Toolbar depends on ModelReady to load

* Change attachTo to wait for ModelReady like KenelDropDown

* sanitizeSavedKernelInfo to fix invalid kernel and display_name. For example: PySpark1111 and PySpark 1111


* Added _contextsChangingEmitter to change loadContext msg when changing kernel

* Resolve PR comments

src/sql/workbench/services/notebook/common/notebookRegistry.ts

@@ -52,6 +52,9 @@ let notebookProviderType: IJSONSchema = {
 						name: {
 							type: 'string',
 						},
+						displayName: {
+							type: 'string',
+						},
 						connectionProviderIds: {
 							type: 'array',
 							items: {