`InnerFlowId` should be a per-`NetworkImpl` concept

[FelixMcFelix]

opte/lib/opte/README.adoc

Lines 109 to 113 in 5470aa1

	`InnerFlowId`:: The inner flow ID is the 5-tuple of the inner packet,
	i.e. the packet as an Oxide VPC guest sees it. This is tightly coupled
	with the `oxide-vpc` network implementation. Ideally, the flow ID
	definition would be generic in `opte`, and defined by the network
	implementation.

We don't yet have any other NetworkImpl, much less one that would require a different FlowId type. Plumbing this through would be quite an involved task. When we do so, we want it to use the trait FlowId introduced in #990 -- at that time we should also move the trait into opte_api.