Make DEFAULT_SERVICE supported behaviour

[erkghlerngm44]

So the package can be restricted to use one service only, so the service won't have to be specified on every request.

Currently it defaults to ANILIST while showing a warning, if no service is specified.

Maybe some examples:

import aniffinity
aniffinity.endpoints.DEFAULT_SERVICE = "MYANIMELIST"
aniffinity.Aniffinity("erkghlerngm44")
...

or possibly specified in the Aniffinity class?

import aniffinity
af = aniffinity.Aniffinity("erkghlerngm44", base_service="ANILIST", ...something)

Or possibly something else?