add missing L4Protocol values for IPv6 extension headers

[dmitris]

I believe a few values for IPv6 extension headers [1] are missing in the current L4Protocol https://docs.rs/network-types/0.0.1/src/network_types/l4/mod.rs.html#7-39 values and can be added. (For reference, the full list of the "Assigned Internet Protocol Numbers": https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml)

[1] https://www.iana.org/assignments/ipv6-parameters/ipv6-parameters.xhtml

[svencowart]

@vadorovsky — you can close this issue. I've verified that all IPv6 extension headers are available in the latest version. Here is the list:

IPv6 Ext Proto	ip.rs line #
0	217
43	303
44	305
50	317
51	319
60	337
135	487
139	495
140	497
253	507
254	509