Prepend JSON [ before encoding events in HttpSink to avoid relocating bytes

[pablosichert]

When the JSON encoding is selected the http sink prepends [ to the body after encoding events. To prepend to the byte buffer, memory of the whole body needs to be relocated or reallocated.

This could be avoided by introducing e.g. a prepare_body method that writes data to the empty body before encoding events.

Also see discussion in #11232 (comment) where this behavior has been resurfaced.