* Add dns64 plugin

Add external plugin to core in-tree.
* Pull code from upstream: https://github.com/serverwentdown/dns64
* Update docs.
Signed-off-by: Ben Kochie <superq@gmail.com>

* Make dns64 consistent.
Signed-off-by: Ben Kochie <superq@gmail.com>

* Cleanup README
Signed-off-by: Ben Kochie <superq@gmail.com>

* Cleanup minor issues.
Signed-off-by: Ben Kochie <superq@gmail.com>

* Remove proxy method.
Signed-off-by: Ben Kochie <superq@gmail.com>

* dns64: big cleanup

* Make the code a bit more idiomatic
* Add tests
* use proper Upstream API
Signed-off-by: Casey Callendrello <c1@caseyc.net>
Signed-off-by: Ben Kochie <superq@gmail.com>

* A little more clenaup

* Fix some docs.
* Use the correct plugin register method.
* Cleanup some review items.
Signed-off-by: Ben Kochie <superq@gmail.com>

* Add metrics counter for DNS64 translations

Add a basic counter of how many DNS64 translations have been completed.
Signed-off-by: Ben Kochie <superq@gmail.com>

* Add DNSSEC bug link
Signed-off-by: Ben Kochie <superq@gmail.com>

* Test cleanup
Signed-off-by: Ben Kochie <superq@gmail.com>

* dns64: more test cleanup
Signed-off-by: Casey Callendrello <c1@caseyc.net>
Co-authored-by: Casey Callendrello <c1@caseyc.net>