[7.1.0] Implement a new execution log conversion tool. (#21192)

For now, it knows how to convert between the binary and JSON formats.
Ability to convert from the new compact format will be added in a
followup.

Tested by running manually. I will add end-to-end test coverage once the
converter has been fully implemented.

PiperOrigin-RevId: 603684017
Change-Id: Idf0d851feb9ea22e7021a1b62f003f0978bcd378