[ENHANCEMENT] Add way to specify user agent and change default one

[Onako2]

PaperMC has released Fill 3 (the downloads api) recently which will require users to set their user agent.
In order to prevent mscs users from being blacklisted there should be an option to change the user agent and have a default user agent that points to mscs.

Original Discord announcement: https://discord.com/channels/289587909051416579/492517675680006144/1387884496212201742
Docs: https://fill.papermc.io/swagger-ui/index.html#/