Support msgpack Timestamp type in in_forward.

[methane]

Is your feature request related to a problem? Please describe.

Currently, in_forward supports only custom EventTime type for precious timestamp. But msgpack added Timestamp ext-type after forward protocol added EventTime.

Timestamp has these merits:

• It doesn't have 2038 (or 2242 in uint32) year problem.
• Since it is msgpack official, it is more easy and efficient in some languages.

Describe the solution you'd like

in_forward support Timestamp for timestamp.

For compatibility reason, out_forward should not use Timestamp yet. (or opt-in).

Describe alternatives you've considered

Additional context