Option for pretty indented json output

When testing things and debugging, it would sometimes be quite helpful if there was a config option to turn on nicely indented json output. E.g., as you get from Pythons json module via the optional parameters sort_keys and indent to json.dumps.

I started looking into if I could prepare a quick PR myself, but realized it wasn't completely trivial due to the way the body is handled in chunks in optimade/server/middleware.py. A quick (but somewhat ugly) implementation would be to insert something like

if CONFIG.pretty_output:
       content_json = json.loads(content)
       content = json.dumps(content_json, sort_keys=True, indent=4)

but due to the chunking processing I didn't yet find the right place to insert this.

Nevertheless, before I spend more time on it, it would be good to know if others also see this option as useful / if the PR would be welcomed.

Start by reading optimade/server/middleware.py, focusing on how response bodies are handled in chunks and where JSON content is assembled. Check how CONFIG options are defined and consumed. Done means an opt-in configuration setting produces sorted, indented JSON output without breaking chunked response handling.