Move Metatag Asynchronous Widget into Metatag module

Problem/Motivation

Module page says:

The reason for yet-another-module is to get something usable and easy to install & maintain without requiring a quicker release cadence for the Metatag module. If, once the module is feature complete, the maintainers of the Metatag module want to include the widget in the Metatag module itself, then this module will be deprecated.

So let's use this issue to track the integration into Metatags.
Issues like #2935134: [meta] Improve performance of paragraphs widget → show that things can get even worse when using Paragraphs.

Steps to reproduce

Proposed resolution

Move Metatag Asynchronous Widget into Metatags module

Remaining tasks

1. Describe the requirements for an integration into Metatags
2. Resolve the remaining tasks
3. Prepare a MR for Metatags (maybe move this issue over right before)
4. Move Metatag Asynchronous Widget into Metatags module

User interface changes

API changes

Data model changes