refactor: make multipart_form public

[omarabid]

I am implementing a custom connector and this module is necessary in order to implement the connector trait (unless I am doing something wrong?).

This function in particular

    fn http_post_uri_multipart_form<'a, 'p, RS, PT, TS>(
        &'a self,
        full_uri: Url,
        file: Option<FileMultipartData<'p>>,
        params: &'p PT,
        context: SlackClientApiCallContext<'a>,
    ) -> BoxFuture<'a, ClientResult<RS>>

FileMultipartData itself is declared public. However, it is inaccessible behind the private module. Another solution is to re-export it as public.

[abdolence]

Thanks, I have no objections to make it visible if it useful.

[omarabid]

Thanks for the quick release!