TypeVarTuple support

[Zac-HD]

We can't just use Callable[[], Awaitable[T] because we might also accept positional arguments, but either ParamSpec or overloads for zero-to-five positional arguments should work. (the latter because paramspec probably gets kwargs wrong here)

[oremanj]

You need TypeVarTuple to type this correctly. mypy doesn't support it yet: python/mypy#12280

If you use the plugin ([mypy] plugins = trio_typing.plugin) then you should get correct type-checking for these calls. There are a number of different functions in trio that have this signature and I wasn't keen on maintaining N-ary overloads for all of them.