Add User-Agent for subscription requests

[Katze-942]

Well, this tiny PR adds a User-Agent when requesting a subscription URL :x

There are a bunch of reasons for that:

1. Pretty much all proxy clients do this
2. A proxy provider can collect client usage stats
3. Based on the User-Agent, a proxy provider can decide which profiles to serve to that client

So basically, it is just a way to identify what kind of client is trying to fetch the subscription.