feat: Add flatten util

[rklaehn]

I use it all the time to flatten streams, so why not.

in iroh-blobs:

    /// List all complete blobs.
    pub async fn list(&self) -> Result<impl Stream<Item = Result<BlobInfo>>> {
        let stream = self.rpc.server_streaming(ListRequest).await?;
        Ok(flatten(stream))
    }

    /// List all incomplete (partial) blobs.
    pub async fn list_incomplete(&self) -> Result<impl Stream<Item = Result<IncompleteBlobInfo>>> {
        let stream = self.rpc.server_streaming(ListIncompleteRequest).await?;
        Ok(flatten(stream))
    }
    ```

in iroh-docs 3x, 

[rklaehn]

Not worth it I think.