API: only set default content-type when not set

[pzl]

This updates the API methods to only set the default content type (json) when the content-type header has not already been manually specified.

This allows a client to manually specify if they are using a different request body format (e.g. yaml, cbor, smile).

This change is all that was needed to successfully send CBOR from Fleet Server (elastic/fleet-server#1902) into elasticsearch

This (i think) closes #132

[pzl]

FYI the failures here are from some out-of-date API stuff that gets pulled down from make gen-api.

There were a few new files and a couple of parameter updates (and several things moved from experimental to beta). I excluded those things from this PR since they seemed like in-tree updates and chose to kept this PR thematically one-note.

CI didn't like that. Happy to just make an update PR and then rebase this, if that's ok, @Anaethelion

[Anaethelion]

@pzl could you just remove the API generation from the PR and keep only the part within the generator ?
I'll take care of updating the API for main and 8.5 once this is merged!

[Anaethelion]

LGTM!

[Anaethelion]

Tests are failing because API is not in sync anymore with tests, merging and backporting as-is. API will be updated in branch depending on their respective versions.