So far we have used `std::string` for asset catalog paths. Some
operations are better described on a dedicated class for this, though.
This commits switches catalog paths from using `std::string` to a
dedicated `blender::bke::AssetCatalogPath` class.
The `using CatalogPath = AssetCatalogPath` alias is still there, and
will be removed in a following cleanup commit.
New `AssetCatalogPath` code reviewed by @severin in D12710.
628fab696c