'az extension add --name' should output standard log to stdout

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.5.1

**Describe the bug**

Currently when using 'az extension add --name <>', the following output will show in stdout instead of stderr
- Searching ..
- Downloading..

In some case we want to disable stdout to decrease verbose:
az extension add --name <> > /dev/null

But message lik '-- Searching ...' is still showing up.

we would not redirect stderr to /dev/null since we would like to see message if error really occurs.

**To Reproduce**
az extension add --name azure-devops > /dev/null

**Expected behavior**
Only exception is show in stderr, install progress shoud appear in stdout

**Environment summary**

**Additional context**