Treat IPv6 Routing Header separately and incorporate diff Routing Types

[zeeshanlakhani]

Background

Currently, we treat the Segment Routing Header (SRH) as the only type of routing header that's possible, with its suggested routing type of 4, as per the spec. However, looking at the RFC for the IPv6 Routing Header, it's possible for more type-specific data and different routing types to exist, of which an SRH would just one kind of type-specific data. We should also accommodate to those who want to add their own specific, custom routing types for example.

Possible Solution

As @drunkirishcoder has shown with the current WIP https://github.com/capsule-rs/capsule/tree/djin/icmp-redux, we can treat different routing types somewhat similarly to how we're handling icmp messages.