support `az sig exists`

[Jinnan-Guo]

Related command
az sig **exists**
az sig image-definition **exists**

Is your feature request related to a problem? Please describe.

I would like to check whether an image gallery exist. I could use the existing az sig show to check.

If a gallery exist, it (az sig show) send output to stdout. If a gallery does not exist, it send error msg to stderr. A simple method like az sig exists that send a simple json result to stdout in both case would make scripting easier.

Describe the solution you'd like

Create an exists method for az sig commands. Return a json (e.g., {"exists": "false"}) to stdout.

Similar to existing az cli command e.g., az storage container exists

Describe alternatives you've considered

Additional context

[yonzhan]

Thank you for opening this issue, we will look into it.