Add VendorSpecificInfo from RFC 2132

[leshow]

We need a more robust way to handle vendor specific information from https://www.rfc-editor.org/rfc/rfc2132#section-8.4

Right now dhcproto just stores the section as a big block of bytes, there is no way to pull out sub-options of vendor info, that's left as an exercise to the user.

I'm happy to do this, but I thought someone might want to take a crack at it or have thoughts. Here are the kea docs on it, the "notes" section at the bottom of this is helpful https://kea.readthedocs.io/en/kea-1.6.2/arm/dhcp4-srv.html#dhcpv4-private-options

I was thinking we could represent the vendor specific info as HashMap<VendorCode, UnknownOption>. where VendorCode I think would only have the variants Pad End & Unknown. Having the key literally just be u8 may also make sense.