distinguish creation of directories vs. files

[maarten-litmaath]

Copied from #48 (comment) where @paulmillar wrote the essence of the following:

[...]
For example, suppose a token should allow the client to create/upload exactly one file. A token with storage.create:/results/my-result.root would allow the client to create a directory /results/my-result.root and subsequently upload as many files as they want.

An alternative approach would be to use the presence or absence of a trailing slash to indicate what kind of object may be created.

For example, in the scenario where /foo exist as a directory but /foo/bar does not exist, then:

• storage.create:/foo/bar authorises the creation of a file bar but not a directory;
• storage.create:/foo/bar/ authorises the creation of the directory bar but not a file.

Such semantics would first need to be supported in production by data management client MW, before starting to be enforced by storage services, possibly per VO.