Option for custom context on serverless Express server

[dmolin]

When using the servless options, LTIJS defaults to the requester baseUrl and its "/" context when redirecting back to our application. This makes it impossible for this to work in situations where we want LTIjs to use a separate context from the one used by the main web application.

This PR makes it possible to specify a custom context that will be used by the Express server when dealing with LTIjs messaging.

Example usage:

  provider = Provider.setup(
    ltiSignKey,
    {
      url: ltiDB
    },
    {
      // options
      tokenMaxAge: 60 * 60, // 1 hour
      appRoute: "/launch",
      loginRoute: "/login",
      cookies,
      dynRegRoute: "/register",
      dynReg: {
        url: `${settings?.public?.baseUrl}/ltitool`,
        name: `${settings?.private?.lti?.toolName || "LTI Tool"}`,
        description: `${settings?.private?.lti?.toolDesc || "LTI Tool Description"}`,
        redirectUris: [],
        autoActivate: true
      }
    },
    log
  );

  // deploy the LTI provider in serverless mode and bind it to the context '/ltitool'
  await Provider.deploy({ serverless: true, path: "/ltitool" });

This will result in "/ltitool/launch", "/ltitool/login", "/ltitool/register" to be used by the Express server as intended.