Mount HDFS Dialog: basic support (#7580)

Implemented in this PR

- New base dialog for anything needing to work with the controller. This is important since going from SQL -> Controller we "should" have the right permissions but aren't guaranteed
- Support for Mount HDFS via a dialog. Includes basic polling for success/failure, but have to give up after 2.5min as mounting could take hours. By then it's assumed to be successful since server-side has 2min timeout built in.


Not implemented in this PR

- Script as Notebook button. This should convert the inputs to a set of cells in a notebook so users can run things themselves
- Updates based on PM / UX reviews. I think we'll need a round of feedback before completing this work.

extensions/mssql/package.json

@@ -284,7 +284,7 @@
         {
           "command": "mssqlCluster.livy.cmd.submitSparkJob",
           "when": "nodeType == mssqlCluster:hdfs",
-          "group": "1root@1"
+          "group": "1mssqlCluster@7"
         },
         {
           "command": "mssqlCluster.livy.cmd.submitFileToSparkJob",