Add URL equivalent to `$bal_library_dir `

[foundrytom]

What

Add $bal_library_dir that expands to a (percent encoded) file URL pointing to the library directory

Why

This allows the library dir to be used in locatableContent trait location properties that must be a valid, URL-encoded file URL instead of a path. Presently, if the path to the library contains spaces or other non-URL-legal characters, then BAL will return invalid data when $bal_library_dir is used in trait properties that are required to be a URL.

ACs

• file://$bal_library_url/file.ext -> file://c:/path/to/lib/file.ext (Windows) file:///path/to/lib/file.ext (POSIX)
• Path should be normalised: file://$bal_library_url/../file.ext -> file://c:/path/to/file.ext (Windows) file:///path/to/file.ext (POSIX)
• URL should be FileUrlPathConverter::pathFromUrl compatible (e.g. check UNC paths).
• bal_library_dir_url should also be supported
• Normalization behaviour should be triggered based on whether the data is a file url.
• Try very hard to avoid using openassetio utils in BAL backend.

[elliotcmorris]

This thing might be pretty good for testing host integrations.

[feltech]

This came up recently as a blocker for automated testing of a real-world integration.