libdns: Add inwx provider

[onny]

Closes https://github.com/foxcpp/maddy/issues/581

[onny]

@foxcpp When trying to build I get following error:

maddy> Building subPackage ./cmd/maddy
maddy> # github.com/libdns/inwx
maddy> vendor/github.com/libdns/inwx/provider.go:227:13: cannot use record.Priority (variable of type int) as uint value in struct literal
maddy> vendor/github.com/libdns/inwx/provider.go:240:13: cannot use record.Priority (variable of type uint) as int value in struct literal
maddy> # github.com/mattn/go-sqlite3
maddy> sqlite3-binding.c: In function 'sqlite3SelectNew':
maddy> sqlite3-binding.c:128049:10: warning: function may return address of local variable [-Wreturn-local-addr]
maddy> 128049 |   return pNew;
maddy>        |          ^~~~
maddy> sqlite3-binding.c:128009:10: note: declared here
maddy> 128009 |   Select standin;
maddy>        |          ^~~~~~~

Not sure yet how to fix this but maybe I'll have too look into the libdns module system 👀

[foxcpp]

Might be version mismatch between libdns used by certmagic and libdns module.