Validate requested completions against the events supported by an operation

The UPC++ implementation does not detect when a requested completion event is not in the set supported by an operation. For example, the following compiles:

#include <upcxx/upcxx.hpp>

using namespace upcxx;

int main() {
  global_ptr<int> dst;
  rput(3, dst, source_cx::as_future());
}

The spec states that this is UB, but we should really provide a diagnostic, since it's likely something users might do by accident.

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