Add implementation for loading signing keys from Key Vault #533

[berendhaan]

This feature can be enabled by setting TokenSigningConfiguration.UseAzureKeyVault to true and adding the following configuration options in the appsettings.json.

"AzureKeyVaultConfiguration": {
       "KeyVaultUri": "",
        "KeyName": ""
}

Some remarks:

• There is no caching of the keys
• Needs explanation how to configure Key Vault correctly
• Not battle-tested, at the moment max 3 users and signing in a couple of times a week

[skoruba]

Hi @berendhaan - thank you for your PR, I've decided to load certificate from Azure KV on application startup currently - that is the reason why I am closing this PR. Thank you for your effort on this PR.