Make 'az network private-dns record-set a' consistent with 'az network dns record-set a'

**Is your feature request related to a problem? Please describe.**
It seems like it is impossible to create a non-empty A record and set ttl for a private DNS using Azure CLI. If you compare this with the equivalent command for a public DNS, one finds that the `ttl` parameter is available for `az network dns record-set a add-record`. The latter allows us to add a new record with a custom ttl. The equivalent for a private DNS only allows us to create an empty A record with the correct ttl, but does not allow us to add a non-empty A record with a custom ttl.

**Describe the solution you'd like**
Make `az network private-dns record-set a` consistent with `az network dns record-set a`. So add the same parameters, particularly `ttl`.

**Describe alternatives you've considered**
Going through the portal and doing it manually.