Add toString() to Ipv6ForwardingRule

Needed because default Ipv6ForwardingRule#toString doesn't dump
actual data. It is hard to know why two rule comparison failed
in the test failure.

Bug: 250552545
Test: atest BpfCoordinatorTest
Change-Id: Iaf29c888238573c7c9a0a4e0bd65fbe468084e51
2 files changed