As of now, CommunityConfiguration only allows registering a new configuration provider via the attributes section of extension.json. While most CommunityConfiguration clients are likely to be extensions and this registration approach is likely going to be fairly popular, it is not sufficient, because:

• the attributes section is hard to fake from PHPUnit tests within the CommunityConfiguration extension itself,
• the attributes section cannot be used by anything that does not have an extension.json or skin.json file (such as a gadget)

While gadgets are not the primary focus now, we need to implement this at this stage, because this will allow us to write tests with specific providers. Because of that, this task currently blocks T357531: Write an API for manipulating Community configuration.