Add functionapp auth settings

**Is your feature request related to a problem? Please describe.**
There is no way to manage function app auth settings via the CLI. I can't seem to find any way to do it other than through the portal. I've tried to work around it with ARM templates + `az deployment`, and `az resource`, but cannot seem to get it working and there is no guidance offered on how to do so. Building serverless architectures with DevOps practices go hand-in-hand; yet, to enable AAD auth on a function app, I have to go into the portal to manually do it. This is a big challenge in managing these workloads -- spinning up environments is now complex due to _one_ setting. Ephemeral or automated environments are now impossible, we must hand-hold this environment setup (e.g. pets vs cattle).

**Describe the solution you'd like**
Implement `az functionapp auth` in the same fashion as `az webapp auth`: [az webapp auth docs](https://docs.microsoft.com/en-us/cli/azure/webapp/auth?view=azure-cli-latest)

**Describe alternatives you've considered**
I've tried ARM templates, az deployment, az resource, etc. I can't seem to get this to work other than through the portal UI.

**Additional context**
N/A