[mlir:Async] Add the size parameter to the async.group

Specify the `!async.group` size (the number of tokens that will be added to it) at construction time. `async.await_all` operation can potentially race with `async.execute` operations that keep updating the group, for this reason it is required to know upfront how many tokens will be added to the group.

Reviewed By: ftynse, herhut

Differential Revision: https://reviews.llvm.org/D104780