Add support for std::net::Ipv6Addr type

[rcgoodfellow]

IPv6 Address Support

This PR adds support for "ipv6" -> std::net::Ipv6Addr.

I've tested these changes in support of this schema and the generated client is working as expected.

Rust compiler issue

I ran into an issue with the generated code that produced this error.

error[E0004]: non-exhaustive patterns: type `&Null` is non-empty
  --> ddm-admin-client/src/lib.rs:1:1
   |
1  | / progenitor::generate_api!(
2  | |     spec = "../ddm/openapi/ddm-admin.json",
3  | |     inner_type = slog::Logger,
4  | |     pre_hook = (|log: &slog::Logger, request: &reqwest::Request| {
...  |
13 | |     }),
14 | | );
   | |_^ `Null` defined here
   |
   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
   = note: the matched value is of type `&Null`
   = note: references are always considered inhabited
   = note: this error originates in the macro `progenitor::generate_api` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0004`.
error: could not compile `ddm-admin-client` due to previous error

I traced this down to the self dereference in the attached diff. For more details see this issue. I hit this on nightly-2021-11-24 using the 2021 edition.

[ahl]

Nice! Thanks for adding this and thanks for chasing down the mysterious non-exhaustive match issue.