media: always store media to temporary dir

[martinetd]

plain media are no longer accessible easily through non-authentified links, just always download the
thing

Note: the following switch to get_media_file was also implemented, but not kept because get_media_file uses get_media_content + tokio async write internally which is strictly what we are doing...

    async fn to_uri(&self, client: &Client, body: &str, mimetype: Option<&str>) -> Result<String> {
        let Some(dir_path) = &args().media_dir else {
            return Err(Error::msg("<no media dir set>"));
        };
        let media_request = MediaRequestParameters {
            source: self.clone(),
            format: MediaFormat::File,
        };
         let mime = mimetype
             .and_then(|m| m.parse().ok())
             .unwrap_or(mime::APPLICATION_OCTET_STREAM);
        let content = client
            .media()
            .get_media_file(&media_request, None, &mime, false, Some(dir_path.into()))
            .await
            .context("Could not get decrypted data")?;
        let filename = body.rsplit_once('/').map(|(_, f)| f).unwrap_or(body);
        let dir = PathBuf::from(dir_path);
        if !dir.is_dir() {
            fs::DirBuilder::new()
                .mode(0o700)
                .recursive(true)
                .create(&dir)
                .await?
        }
        let file = dir.join(filename);
        content.persist(&file).map_err(|e| e.error)?;
        let url = args().media_url.as_ref().unwrap_or(dir_path);
        Ok(format!(
            "{}/{}",
            url,
            utf8_percent_encode(filename, FRAGMENT)
        ))
     }

Called with:

                .to_uri(
                    matrirc.matrix(),
                    file_content.filename(),
                    file_content
                        .info
                        .as_ref()
                        .and_then(|i| i.mimetype.as_deref()),
                )

Fixes #21