Adds MSAL-Graph auth provider support.

[peombwa]

This PR adds autorization code provider support to handle access token retreival and storage.

// Create session token provider
SessionTokenCacheProvider sessionTokenCacheProvider = new SessionTokenCacheProvider(httpContext);

// Create confidential client application
IConfidentialClientApplication cca = AuthorizationCodeProvider.CreateClientApplication(appId, redirectUri, new ClientCredential(appSecret), sessionTokenCacheProvider);

// Pass authorization code provider to GraphServiceClient
GraphServiceClient graphClient = new GraphServiceClient(new AuthorizationCodeProvider(cca, scopes));

// Make graph request
var me = await graphClient
.Me
.Events
.Request()
.WithUserAccount(ClaimsPrincipal.Current.ToGraphUserAccount())
.GetAsync();