PFX support for az ad sp create-for-rbac

**Related command**
az ad sp create-for-rbac
https://learn.microsoft.com/en-us/cli/azure/ad/sp?view=azure-cli-latest#az-ad-sp-create-for-rbac

**Is your feature request related to a problem? Please describe.**
I want `az ad sp create-for-rbac` to support PFX certificate files.
`az ad sp create-for-rbac` just generates a PEM with `--create-cert`, which is difficult to convert to PFX in Windows PowerShell. It also doesn't accept an existing PFX file for `--cert`, forcing customers to have to convert their certificates to other certificates.

**Describe the solution you'd like**
`az ad sp create-for-rbac` should offer option to generate a PFX file or use an existing PFX file, since customers likely encrypt their certificates.

**Describe alternatives you've considered**
Only way is to install OpenSSL to convert PEM to PFX in Windows PowerShell. Would be much easier if `az ad sp create-for-rbac` supported PFX certificates since customers most likely want to encrypt their certificates anyway.