Add ability to retrieve the ASC secure score via the Azure CLI

**Is your feature request related to a problem? Please describe.**
Currently there is no way to retrieve with the Azure CLI (2.5.1) via the `az security` command group either the old secure score, or the new preview secure score which is visible under the Policy & Compliance section of the Azure Security Center web interface.

**Describe the solution you'd like**
The ability to retrieve your secure score via the `az security` command group.

**Describe alternatives you've considered**
Manually calculate the score using the same method Microsoft describe in their [Enhanced Secure Score (Preview) in Azure Security Center web page](https://docs.microsoft.com/en-us/azure/security-center/secure-score-security-controls).

**Additional context**
N/A