feat(mattermost): add auth and send message to channel action

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/auth/verify-credentials.ts

@@ -0,0 +1,44 @@
+import { IGlobalVariable } from '@automatisch/types';
+import getCurrentUser from '../common/get-current-user';
+
+const verifyCredentials = async ($: IGlobalVariable) => {
+  const oauthRedirectUrlField = $.app.auth.fields.find(
+    (field) => field.key == 'oAuthRedirectUrl'
+  );
+  const redirectUri = oauthRedirectUrlField.value as string;
+  const params = {
+    client_id: $.auth.data.clientId,
+    client_secret: $.auth.data.clientSecret,
+    code: $.auth.data.code,
+    grant_type: 'authorization_code',
+    redirect_uri: redirectUri,
+  };
+  const headers = {
+    'Content-Type': 'application/x-www-form-urlencoded', // This is not documented yet required
+  };
+  const response = await $.http.post('/oauth/access_token', null, {
+    params,
+    headers,
+  });
+
+  const {
+    data: { access_token, refresh_token, scope, token_type },
+  } = response;
+
+  $.auth.data.accessToken = response.data.access_token;
+
+  const currentUser = await getCurrentUser($);
+
+  await $.auth.set({
+    clientId: $.auth.data.clientId,
+    clientSecret: $.auth.data.clientSecret,
+    accessToken: access_token,
+    refreshToken: refresh_token,
+    scope: scope,
+    tokenType: token_type,
+    userId: currentUser.id,
+    screenName: currentUser.username,
+  });
+};
+
+export default verifyCredentials;