[proxies] automatically generate PR when Forge manifest changes

[MikeMatusz]

Describe the feature:
When a new version of Forge is released with updates to the custom-elements.json manifest, it would be nice to automate a PR to forge-angular to pick up any changes. For existing components, running the schematic should generally be sufficient to enable using the new functionality. For new components, more manual intervention is currently required, as the schematic does not currently add the component to the root index or rolled-up modules or create a demo. The PR could potentially be a useful starting point, however, and a prompt that these actions should be taken if anyone wishes to use the component in Angular.

Describe any alternatives you've considered:
The alternative would be a manual process of either Angular developers contributing to Forge following a merged PR up there with another PR here, or Angular developers monitoring Forge for updates in some capacity, perhaps when they wish to update the version of Forge their app is using, and making the necessary updates at that time to keep forge-angular up to date.