add sub_option type & sname/fname to config

[leshow]

We likely need a way to introduce options that have sub-options or "encapsulated options" into the config. Option 43 comes to mind. We support hex decoding, but it is pretty cumbersome to read/write especially in the presence of encapsulated options.

With change, this block of options

  43:
      type: sub_option
      value:
          1:
              type: str
              value: "foobar"
          2:
              type: ip
              value: 1.2.3.4

will be decoded into (it's a hashmap so the order is not consistent)

VendorExtensions: VendorExtensions([2, 4, 1, 2, 3, 4, 1, 6, 102, 111, 111, 98, 97, 114])

I've opened an issue in the protocol repo: bluecatengineering/dhcproto#45 so that we can track supporting encapsulated options better there also. To make VendorExtensions type really usable inside dora we likely need a way to pull out specific sub-options.

I took the opportunity to add sname and fname to the config also. These can now be set with server_name and file_name and will be put in the respective header fields.