provide a method for getting a list of valid license names

[rjbs]

This is not my request, but is transferred from an email I received from @kevindawson.

He would like a way to get a list of valid meta 2 license names from something.

[karenetheridge]

Presently, that's buried in a local variable in CPAN::Meta::Validator, but we could certainly create a new API to return it.

I am curious however what he would use it for -- if he is picturing using it to validate some metadata, it would be better to use the existing CPAN::Meta::Validator methods for that.