Introduce Ipv6UpstreamRule to bpf tethering offload

Replaced startUpstreamIpv6Forwarding and stopUpstreamIpv6Forwarding
with add/remove Ipv6UpstreamRule. This is a preparation for following
CLs which will pass the upstream's prefixes to the bpf map. We might
have more than one upstream rules.

Also renamed Ipv6ForwardingRule to Ipv6DownstreamRule since we have
defined the Ipv6UpstreamRule.

No logic changes are being made in this CL.

Test: atest TetheringTests
Bug: 261923493
Change-Id: I022f97c2daf468bbd4a4279a069ccf498013e7e7