az ssh: Configurable certificate lifetime

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)
`az ssh`

### Description of issue (in as much detail as possible)
As I'm migrating from the legacy [AAD SSH login scheme](https://docs.microsoft.com/en-gb/azure/virtual-machines/linux/login-using-aad), I can't quite understand if/how to configure when the issued SSH certificates expire. I asssume this is (partially) configured in Azure AD but I'm lacking the detailed knowledge on where to configure this.

More details in the [relevant documentation](https://docs.microsoft.com/en-gb/azure/active-directory/devices/howto-vm-sign-in-azure-ad-linux#using-conditional-access) would be greatly appreciated in addition to possibly more flags in `az ssh` to configure SSH certificate lifetime.