feature: Radiustar::Dictionary#read_xml

[gderosa]

Being able to interpret "dictionary" files like this would be really cool :

https://www.iana.org/assignments/radius-types/radius-types.xml

mainly because these data represent the official standard from IANA.

Well, there is actually a plain text format from the IANA website, but it's very different from FreeRADIUS-style dictionary files that Dictionary#read is currently able to parse. Here it is:

https://www.iana.org/assignments/radius-types/radius-types.txt