az cloud register does not support a way to set MHSM dns suffix

'az cloud register' command allows a user to set a new Azure cloud environment. A new DNS Suffix for Managed HSM is now present in standard cloud definitions.

% az cloud show --query suffixes.mhsmDns
".managedhsm.azure.net"

However using 'az cloud register' it is not possible to set the DNS suffix for Managed HSM.
There exists a --suffix-keyvault-dns, but there is no '--suffix-mhsm-dns' parameter supported. It needs to be added.

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az cloud register`

**Errors:**

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az cloud register --help`

## Expected Behavior

## Environment Summary
```
Windows-10-10.0.19041-SP0
Python 3.6.8
Installer: MSI

azure-cli 2.20.0

```
## Additional Context