trace2: implement trace2_printf() for event target

trace2: implement trace2_printf() for event target

The trace2 event target does not have an implementation for
trace2_printf(). While the event target is for structured events, and
trace2_printf() is for unstructured, human-readable messages, it may
still be useful to wrap these unstructured messages in a structured JSON
object. Among other things, it may reduce confusion when manually
debugging using event trace data.

Add a simple implementation for the event target that wraps
trace2_printf() messages in a minimal JSON object. Document this in
Documentation/technical/api-trace2.txt, and bump the event format
version since we're adding a new event type.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>