Stream .rrd files loaded over http

[emilk]

We currently download the entire .rrd before we start parsing it. This means it can be a long delay before a user sees anything on e.g. https://demo.rerun.io

We are using my ehttp crate (since it works on both native and on wasm), so either we find an alternative to it, or we improve it to support chunked downloads.

Our re_log_encoding crate already supports streaming decoding of the .rrd file format.