Add initial Microsoft Teams support

Documentation on https://github.com/42wim/matterbridge/wiki/MS-Teams-setup

vendor/github.com/yaegashi/msgraph.go/msauth/client_credentials_grant.go

@@ -0,0 +1,27 @@
+package msauth
+
+import (
+	"context"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/clientcredentials"
+	"golang.org/x/oauth2/microsoft"
+)
+
+// ClientCredentialsGrant performs OAuth 2.0 client credentials grant and returns auto-refreshing TokenSource
+func (m *Manager) ClientCredentialsGrant(ctx context.Context, tenantID, clientID, clientSecret string, scopes []string) (oauth2.TokenSource, error) {
+	config := &clientcredentials.Config{
+		ClientID:     clientID,
+		ClientSecret: clientSecret,
+		TokenURL:     microsoft.AzureADEndpoint(tenantID).TokenURL,
+		Scopes:       scopes,
+		AuthStyle:    oauth2.AuthStyleInParams,
+	}
+	var err error
+	ts := config.TokenSource(ctx)
+	_, err = ts.Token()
+	if err != nil {
+		return nil, err
+	}
+	return ts, nil
+}