Utility improvements for handling V6 encap and XFRM_MIGRATE

This commit is a preparation CL for testing the ability to update
IpSecTransform with new addresses. This commit adds three supports
in utility classes:
- Support verifying outbound v6 encap packets
- Allow awaitEspPacketNoPlaintext to wait for a packet with expected
  length until timeout. Previously this method would fail
  immediately when it received an ESP packet with incorrect length.
  This old behavior would make the tests flaky since the test device
  might send ESP containing RA message instead of the test data.
- Support checking the existence of XFRM_MIGRATE feature

Bug: 266149275
Test: atest CtsNetTestCases
Change-Id: Ia50aeda8d134c5edd2d1b86bf43a521084ce9866