support `az sig exists`

**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**