GRE dissector for IPGRE/IP6GRE on LINUX_SLL/SLL2 #2501

Summary

Part of #2460. LinuxSLL/LinuxSLL2 fall back to Raw for the IPGRE
(778) and IP6GRE (823) arphrd_type values, whose protocol_type field
isn't an EtherType (see _ARPHRD_NOT_ETHERTYPE in
pcapkit/protocols/link/linux_sll.py). The header is preserved, nothing
crashes, but nothing is dissected either.

Scope

GRE over IPv4 and GRE over IPv6 respectively -- no GRE dissector exists yet
in this codebase at all (confirmed: no pcapkit/protocols/*/gre.py). The
GRE header itself (RFC 2784, with the optional checksum/key/sequence fields
of RFC 2890) is identical regardless of which IP version carries it; what
differs between IPGRE and IP6GRE is only which outer IP version the
Linux kernel's GRE tunnel interface reports via arphrd_type, not the GRE
framing itself.

So this is naturally one dissector, two arphrd_type bindings:

• a GRE class under pcapkit/protocols/link/ (or wherever the
  layer-placement convention lands it once drafted) implementing RFC
  2784/2890's header: flags, version, protocol type (an EtherType, selecting
  the tunnelled payload), and the optional checksum/offset, key and sequence
  number fields gated by the flag bits
• LinuxSLL's arphrd_type dispatch gets both ARPHRD.IPGRE and
  ARPHRD.IP6GRE wired to the same class, the way FRAD (Dissectors for non-ethertype ARPHRD types on LINUX_SLL/SLL2 (NETLINK/IPGRE/IP6GRE/802.11 radiotap/FRAD) #2460) added a
  single new dispatch branch

Not implemented here -- tracked as its own issue per #2460's scoping.