Add Azure Vault Support

[kdcllc]

Add Azure Vault Key Configuration provider that can be used with MSI and without MSI token provider.

-How to use managed identities for App Service and Azure Functions

-Add method to get an access token

-Using Azure KeyVault to manage secrets of eShopOnContainers

-Connection String Support

Everything can be done thru:

<PackageReference Include="Microsoft.Extensions.Configuration.AzureKeyVault" Version="2.2.0" />

This what the alternative code would look like:

       var kv = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(GetToken));
        configBuilder.AddAzureKeyVault(options.BaseUrl, kv, new DefaultKeyVaultSecretManager());

        public static async Task<string> GetToken(string authority, string resource, string scope)
        {
            var options = Configuration.Bind<AzureVaultOptions>("AzureVault");

            var authContext = new AuthenticationContext(authority);
            ClientCredential clientCred = new ClientCredential(options.ClientId, options.ClientSecret);
            AuthenticationResult result = await authContext.AcquireTokenAsync(resource, clientCred);

            if (result == null)
                throw new InvalidOperationException("Failed to obtain the JWT token");

            return result.AccessToken;
        }

Issue of using outside of MSI

Get access token using Microsoft.Azure.Services.AppAuthentication in a container