Implement KeyVault dataplane access

[benc-uk]

Secrets will need to be fetched, as part of #21

• [ ] Investigate Go options, SDK
• [ ] How we will authenticate
• [ ] Ensure authentication works along side current Terraform and Azure API auth
• [ ] Create wrapper / helper package to get secrets specifically for rover use case

[sebastus]

re: investigate go options, SDK. There are several. I took the most recent.

re: how will we authenticate. We took a decision to authenticate via the az cli. This is how it's done in the sdk: authorizer, err := auth.NewAuthorizerFromCLIWithResource(audience) where audience is the resource url for keyvault (e.g. https://vault.azure.net)

re: ensure authentication works... Well, yes.

re: create wrapper, etc. GetSecret() supports 99% of use cases. FindKeyVault() is used to find the right vault based on tags & subscription.