Entity containers for member type

[bjarnef]

Content types (document, media and member) has been more streamlined compared to previous versions of Umbraco.
Folders are mainly used for document types / element types, an option for media types as well (although I don't think it is used in most projects and not by default and it isn't currently an option to structure member types into folder, although member types now have other features like composition, copy etc.

Ideally I think document, media and member types can inherit a base ContentType controller or similar to re-use same functionality.
The current code base have a EntityContainerRepository but MemberTypeContainerRepository didn't implement this like document and media content types.

With Umbraco Commerce it could add a bigger need for this, allowing us to structure member types in folders as well.

Suggested PR for current/legacy Umbraco: #14833

[bjarnef]

@mattbrailsford maybe something that may make sense in large commerce projects using Umbraco Commerce or @JeffreyPerplex in uMarketingSuite?