Notebooks: Save cell connection name in cell metadata (#13208)

* save connection info in notebook metadata

* update attachTo dropdown based on saved alias

* add setting for saving connection (default=false)

* save/read cell connection name to/from metadata

* get started on toggling multi connection mode

* add activeConnection property to cell model

* add changeContext method for cell

* add comments

* add unit test for reading connection name

* save connection mode in metadata

* clean up code

* address PR comments

src/sql/azdata.proposed.d.ts

@@ -81,6 +81,10 @@ declare module 'azdata' {
 		export interface INotebookMetadata {
 			connection_name?: string;
 		}
+
+		export interface ICellMetadata {
+			connection_name?: string;
+		}
 	}
 
 	export type SqlDbType = 'BigInt' | 'Binary' | 'Bit' | 'Char' | 'DateTime' | 'Decimal'