az KeyVault secret download should be able to send to stdout

**Is your feature request related to a problem? Please describe.**
I would like the ability to pipe a downloaded secret to another command. For instance, if I download a PFX certificate, I'd like to be able to pipe the PFX to an openssl command to convert it to a PEM file.

**Describe the solution you'd like**
Perhaps allow the -f option to accept a special value that would indicate it should stream output to stdout.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.