Add azure auth backend

[Pale-whale]

Hello !

First of all, thanks for this project ! We were having problems with argocd syncing mutated secrets with our ancient controller and this will be an amazing piece of code to run in our clusters :)
But since we are on azure and use managed aks we also use the Managed System Identities to authenticate against the Vault API, so the aim of this PR is to be able to get this identity on aks and send the token to the vault

I've basically copy/pasted the kubernetes auth method and changed the jwt retrival with a curl to azure services.
I'm quite not sure about the name of the environment variables, I tried to keep a cohesion with what was existing

I've updated the chart and the readme accordingly but if there's anything wrong I can correct that :)

Also this support only managed system identities present on azure VM and VMScaleSet Microsoft doc

Thanks :)

[ricoberger]

Hi @Pale-whale thanks for your contribution.

Your contribution looks good to me, but unfortunately I haven't a Vault instance with an Azure backend. I would create a new release with your addition. If you see any problems it a follow up PR would be highly appreciated.