Feature: Add IPv6 support

[tvdijen]

Closes #26

[tvdijen]

@dapphp Is this something you would consider merging?

[dapphp]

Thanks for proposing this long ago. Way back then I had some outstanding changes to support the IPv6 data types for attributes which are now available in the 3.1.0 release. I also borrowed inspiration from this code to better support IPv6 server addresses passed to the constructor and/or setServer().

It was always possible to talk to an IPv6 server by passing a bracketed IPv6 address or if the hostname resolved to an IPv6 address through the system, but again, based on inspiration from your code I added better IPv6 support.

See https://github.com/dapphp/radius/blob/master/README.md#ipv6-server-support and https://github.com/dapphp/radius/blob/master/README.md#mixing-ipv4-and-ipv6-addresses-in-a-clustered-setup for more.

Thank you and all the best to you.