out_http: Use x-ndjson content type for json_lines

[mrueg]

json_lines currently results in Content-Type application/json, but it should be application/x-ndjson.

Fluentd uses application/x-ndjson
https://github.com/fluent/fluentd/blob/284bf4064a9831f5262330a94e3505e09ef6a068/lib/fluent/plugin/out_http.rb#L192

This spec here suggests the same: https://github.com/ndjson/ndjson-spec?tab=readme-ov-file#33-mediatype-and-file-extensions

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[mrueg]

@edsiper would you be able to have a look at this one?

[cosmo0920]

This could be good to change for us. Containing newlines JSON should be treated as x-ndjson.