This restores a basic version of the `post` command.
Some source types have been omitted from this first take.
I copied from `fetch` and from `post`@0.40.0.
Part of #4356
* Move fetch to extra
* Move byte stream code to a function instead of copying it twice
* Fix formatting issues
* Make fetch a default command
* Fix formatting
* Port fetch to engine-q
* Fix check for path as a string
* Add a timeout flag and fix some span issues
* Add a temporary fetch command that returns byte streams. Got rid of async stuff as we're using the blocking feature of tokio
* More tweaks for the bytestream
* Rewrite fetch using ByteStreams
* buffer read on bytes directly
Co-authored-by: Stefan Stanciulescu <contact@stefanstanciulescu.com>
