Port autoscaling to C# (#2296)

Co-authored-by: stas <statis@microsoft.com>

src/ApiService/FunctionalTests/Auth.cs

@@ -0,0 +1,63 @@
+using Microsoft.Identity.Client;
+
+namespace Microsoft.Morse;
+
+public record AuthenticationConfig(string ClientId, string TenantId, string Secret, string[] Scopes);
+
+interface IServiceAuth {
+    Task<AuthenticationResult> Auth(CancellationToken cancelationToken);
+}
+
+public class ServiceAuth : IServiceAuth, IDisposable {
+    private SemaphoreSlim _lockObj = new SemaphoreSlim(1);
+    private AuthenticationResult? _token;
+    private IConfidentialClientApplication _app;
+    private AuthenticationConfig _authConfig;
+
+    public ServiceAuth(AuthenticationConfig authConfig) {
+        _authConfig = authConfig;
+
+        _app = ConfidentialClientApplicationBuilder
+                .Create(authConfig.ClientId)
+                .WithClientSecret(authConfig.Secret)
+                .WithTenantId(authConfig.TenantId)
+                .WithLegacyCacheCompatibility(false)
+                .Build();
+    }
+
+    public async Task<AuthenticationResult> Auth(CancellationToken cancelationToken) {
+        await _lockObj.WaitAsync(cancelationToken);
+        if (cancelationToken.IsCancellationRequested)
+            throw new System.Exception("Canellation requested, aborting Auth");
+
+        try {
+            if (_token is null) {
+                throw new MsalUiRequiredException(MsalError.ActivityRequired, "Authenticating for the first time");
+            } else {
+                var now = System.DateTimeOffset.UtcNow;
+                if (_token.ExpiresOn < now) {
+                    //_log.LogInformation("Cached token expired on : {token}. DateTime Offset Now: {now}", _token.ExpiresOn, now);
+                    throw new MsalUiRequiredException(MsalError.ActivityRequired, "Cached token expired");
+                } else {
+                    return _token;
+                }
+            }
+        } catch (MsalUiRequiredException) {
+            //_log.LogInformation("Getting new token due to {msg}", ex.Message);
+            _token = await _app.AcquireTokenForClient(_authConfig.Scopes).ExecuteAsync(cancelationToken);
+            return _token;
+        } finally {
+            _lockObj.Release();
+        }
+    }
+
+    public void Dispose() {
+        ((IDisposable)_lockObj).Dispose();
+        GC.SuppressFinalize(this);
+    }
+
+    public async Task<(string, string)> Token(CancellationToken cancellationToken) {
+        var t = await Auth(cancellationToken);
+        return (t.TokenType, t.AccessToken);
+    }
+}