chore: content-api response format header

[alexandrebodin]

What does it do?

Add a header to get a response format following v4 old shape to ease v5 migration.

Why is it needed?

To help with migrating to v5

How to test it?

Call the content-api (only affects core-api) with a Strapi-Response-Format: v4 header to get the old format

Related issue(s)/PR(s)

Let us know if this is related to any issue/pull request

[joshuaellis]

Should we be logging a warning if you use the v4 system and flag it as deprecated?

[alexandrebodin]

Should we be logging a warning if you use the v4 system and flag it as deprecated?

I don't see a good way to log a deprecation here. It's going to be used on the client side and not the server side where it would be logged.
it would also log on each request which is not ideal and I'm not convinced it's worth managing state to log it only once in a while.

And It's probably only going to be mentionned in the migration guide