refactor: move media type logic to Plan module

[steve-chavez]

Part of #2825.

[steve-chavez]

According to the loadtest results we lose some throughput with this change https://github.com/PostgREST/postgrest/pull/2934/checks?check_run_id=16660131983

param	head	main
throughput	307.90351639998954	315.3078228340249

Which is weird because it's the same logic, just on a different module. I've seen this case before, will check what's wrong with the Plan module.

[wolfgangwalther]

Remember, a single CI run can always be just random noise. For example you could have had more load on the machine that the job is running on during the "main" part of the test, but not during "head".

Always repeat those tests locally, while keeping load constant. I just did and got:

• main: 449.36
• your branch: 449.20

I don't think there is anything wrong here.

[steve-chavez]

Thanks a lot for the help Wolfgang,

Always repeat those tests locally, while keeping load constant.

Yeah, actually I did that and I always got worse results for this PR. Now, after #2940, I don't get bad results, in fact they seem slightly better:

• main: 565.19
• this branch: 574.37

Maybe there was some perf improvement on #2940 after all. Anyway, this is looking good to merge now. Postponing this change will make #2825 harder otherwise.