InputJsonParser: Add encodeEvent API for symmetry

Add an API to be able to encode events in the parser's format, which
will remove the need for us to hand-write the JSON files in tests.

Bug: 310997010
Test: Presubmit
Flag: TEST_ONLY
Change-Id: Iccb3b94f49c6453931d1a895d91ef032519d7dc2