[rust-server] Cope with base paths with trailing '/'

Created by: bjgill

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Description of the PR

Since we're concatenating URL path segments as strings, we need to strip any trailing '/'.

Of course, the correct fix would be to use the Url type and perform the concatenation correctly. This is just a quick fix to ensure that rust-server stops mangling the URL when no base_path is provided (we've been exposed by the recent update to swagger-parser - see #951).

I've also renamed base_path to base_url in a file where it was actually a URL being referenced, and tidied up some imports.

The renaming is a change to the public API. However, it's fully back-compatible.