Using `urljoin` wth an absolute path in the OFREPProvider prevents connection to self-hosted APIs with OFREP API not at base / path

[atmask]

Description: Operations teams often host feature flagging services themselves rather than using a SaaS solution. In this instance they put their api under a path other than the base / path. The expected behaviour when using the OFREP Provider would be that I pass my base_url (i.e. https://mydomain.com/tools/featureflags) and all subsequent requests would be made using the defined base_url.

The actual behaviour is that the urljoin("https://mydomain.com/tools/featureflags", f"/ofrep/v1/evaluate/flags/{flag_key}") strips the path from the base_url and produces a request to https://mydomain.com/ofrep/v1/evaluate/flags/{flag_key}. This is because the second param in urljoin is being passed as an absolute path that starts with "/"

Proposed Solution: Convert the second arg of urljoin to a relative path and add logic to ensure that the first args ends with /

            evaluation_base_url = self.base_url if self.base_url.endswith("/") else f"{self.base_url}/"
            response = self.session.post(
                urljoin(evaluation_base_url, f"ofrep/v1/evaluate/flags/{flag_key}"),
                json=_build_request_data(evaluation_context),
                timeout=self.timeout,
                headers=self.headers_factory() if self.headers_factory else None,
            )