Add support for path in base url

Currently, if you are using a base_url with a path inside it (e.g: https://gitlab.com/gitlab-org/gitlab/-/raw/v17.11.6-ee/doc/api/openapi/openapi.yaml will documents server.url as https://www.gitlab.com/api/v4), the call to Url::join with the documented path (e.g: /groups/{id}/badges) will incorrectly produce an final url of https://www.gitlab.com//groups/{id}/badges, because of the (I find very very surprising) behavior of Url::join https://docs.rs/url/latest/url/struct.Url.html#method.join

This fixes adds a trailing '/' to the base_url field upon creation, then strips the starting '/' of the path found in the openapi spec so to produce a correct final url.

If this is acceptable for you, I'll happily rebase in a single commit and remove the Cargo lock change too before merging.

PS: great job on the tool!