[Pendo] initialize pendo with segment anonymous_id

When using pendo through segment, the segment anonymous id does not always reach pendo. I am not entirely sure what the issue in this library is, but it is likely related to issues: #317, #325, and #349.

It seems like the pendo integration is initialized before segment. I came to this conclusion after seeing this comment

With AJS classic, we allow users to call setAnonymousId before the library initialization.
This is important because some of the destinations will use the anonymousId during the initialization,
and if we set anonId afterwards, that wouldn’t impact the destination.

Also Ensures events can be registered before library initialization.
This is important so users can register to 'initialize' and any events that may fire early during setup.

in analytics-next library and then attempting to call setAnonymousId before segment is initialized. This change should ensure that it is flushed and create a segment anonymous id before the integrations are loaded/initialized however even after this change the pendo destination was not receiving that segment anonymous id even though it did exist.

A temporary fix for this is hooking into segments 'initialize' event and forcing the expected pendo behavior (the stuff that the pendo integration does on identify):

   // analytics.js snippet file
    analytics.setAnonymousId();

    analytics.on("initialize", function () {
      // we want to make sure that we use segments pendoified anonymous id or
      // segments user id
      var isUserAnonymous = !this._user.id();
      var id = isUserAnonymous
        ? "_PENDO_T_" + this._user.anonymousId()
        : this._user.id();

      // pendo options only requires visitor/account metadata
      var options = {};
      var visitor = Object.assign({ id: id }, this._user.traits());
      options.visitor = visitor;
      var account = Object.assign(
        { id: this._group.id() },
        this._group.traits()
      );
      options.account = account;

      // ensure that on initialize pendo will have segment data and that if
      // segment data already exists it is not overwritten
      window.pendo_options.visitor = Object.assign(
        options.visitor,
        window.pendo_options.visitor
      );
      window.pendo_options.account = Object.assign(
        options.account,
        window.pendo_options.account
      );
    }); 
    
    analytics.load()

Start by reading the referenced analytics-next browser initialization flow and the Pendo integration in integrations/pendo/lib/index.js, especially its identify behavior. Reproduce initialization through Segment with an anonymous_id, then trace whether Pendo receives it before integrations load. Done means Pendo receives the Segment anonymous ID during initialization without overwriting existing Pendo visitor or account data.