Introduce LTTNG_KERNEL_SESSION_SET_CREATION_TIME

Introduce LTTNG_KERNEL_SESSION_SET_CREATION_TIME

Add trace_creation_datetime to the metadata env field

This allows a viewer to get more information regarding the creation time
of a trace. This information is normally inferred from the trace
hierarchy.

The ABI expects an ISO8601 compliant string value. We leave the
formatting to the trace controller.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>