fix(swagger-ui): Use relative redirect to add trailing slash

[boblehest]

Given SwaggerUi::new("/path"), we would previously redirect requests for /path to /path/ (adding the trailing slash). Using an absolute redirect path like this can cause issues when the HTTP server is behind a proxy doing path rewriting.

Example scenario detailing the issue:

Say you have a Rust application which exposes a HTTP server with utoipa-swagger-ui at path /path/to/swagger/. The server is available at hostname my.server.

You also have a proxy server at my.proxy, which handles incoming requests to the path /api/... by stripping the path prefix /api, and forwarding the request to my.server.

Then you do the following:

1. You send a request to my.proxy/api/path/to/swagger
2. The server at my.proxy forwards the request to my.server/path/to/swagger (note the stripped /api prefix)
3. The server at my.server redirects it to /path/to/swagger/ (to "add a trailing slash")
4. The client then follows this redirect, sending a new request to my.proxy/path/to/swagger/
5. The request fails, because the proxy does not serve anything at this path (the path has no /api/ prefix).

Solution:

Redirecting to the relative path swagger/* should be more robust, as it more precisely expresses the intent of simply adding a slash to the end of the path, instead of replacing the entire path.

*Or more generally, redirecting to X/ where X is the last path segment of the configured swagger-ui path.

[boblehest]

There are some previous issues which discuss serving SwaggerUi behind a proxy: #842 and #856 .

I've personally had two issues when trying to serve it being a proxy that does path rewriting.

This PR fixes one of them (people forgetting to add the trailing slash, getting redirected to a broken URL, then coming to ask me why my API doesn't host SwaggerUi).

The other problem I've run into is SwaggerUi not finding openapi.json, which I work around by following the hint from this comment in issue 856:

SwaggerUi::new("/swagger-ui")
    .url("/api-docs/openapi.json", ApiDoc::openapi())
    .config(utoipa_swagger_ui::Config::new(["../api-docs/openapi.json"])),
                                           // ^-- override to use relative path