Dagster-graphql doesn't support --path-prefix

[NakulK48]

Summary

In the client https://github.com/dagster-io/dagster/blob/master/python_modules/dagster-graphql/dagster_graphql/client/client.py#L72

        self._url = (
            ("https://" if self._use_https else "http://")
            + (f"{self._hostname}:{self._port_number}" if self._port_number else self._hostname)
            + "/graphql"
        )

If dagit is instead running at https://host:3000/dagster, there is no way of communicating this to the Graphql client.

Reproduction

Dagit UI/UX Issue Screenshots

Additional Info about Your Environment

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[yuhan]

Hi @NakulK48, thanks for opening this issue. Agreed that a --path-prefix arg would be great. Is it something you like to open a PR for? We'd definitely welcome a contribution.