[META] Support grouping of config entities

After 280+ comments in a solution-searching thread, we finally found both an approach and sponsor to allow Group to support the grouping of config entities. This thread will serve to group all of the individual tasks so that we can keep a nice overview of progress.

Here is the summary from that other issue:

1. We create a content entity type called ConfigWrapper (group_config_wrapper)
2. This entity type has almost zero UI, routes, operations, etc.; it only exists in code
3. If you want to be able to group certain config, you write a plugin that targets said config entity type
4. Then, for each plugin targeting a config entity type, we automatically create a bundle for ConfigWrapper
5. Behind the scenes, we secretly wrap the config entity in order to add it to a group, but as far as handlers are concerned, they're dealing with the actual config entity
6. We do not write a UI yet that allows you to add config entities to a group, modules for Menu, Webform, etc. will have to write their own UI
7. Third-party code should be almost oblivious to the fact that wrappers exist
8. Query access checks don't even run because config entity types don't use query access
9. Regular access checks work the same as before because we'll adapt the group content storage to deal with wrappers, so loadByGroup() et al will still work as before

This should lead to minimal impact and actually be fully backwards compatible. The only major change will happen in the group content storage and people directly dealing with queries on any GroupContent table will need to make their code aware of wrappers.

Some important things of note:

• The wrapper would literally be that, a reference to a config entity
• It would not have any other fields
• If the target config entity is removed, so is the wrapper and GroupContent entity
• If the GroupContent entity is removed, the wrapper remains

In essence: A wrapper cannot exist without wrapped config, but a wrapper existing does not necessarily mean the config entity is grouped.

All contributors to the other issue will be credited here.