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/sqlops.proposed.d.ts

@@ -2050,6 +2050,7 @@ declare module 'sqlops' {
 
 		export interface IStandardKernel {
 			readonly name: string;
+			readonly displayName: string;
 			readonly connectionProviderIds: string[];
 		}