Fix #5874: Use of designated initializers fails Windows build

[definability]

Designated initializers are a feature of C++20:
https://en.cppreference.com/w/cpp/language/aggregate_initialization. However, they appeared in

mediapipe/mediapipe/framework/output_stream_handler.cc

Lines 145 to 149 in ffe429d

	monitoring_info_vector.emplace_back(OutputStreamMonitoringInfo(
	{.stream_name = DebugStreamName(id),
	.num_packets_added = stream->NumPacketsAdded(),
	.next_timestamp_bound = stream->NextTimestampBound()}));
	}

The project uses C++17 and adapting it for C++20 would be non-trivial. The easiest fix is to avoid using aggregated initialization. It is fine while the order of members is preserved:

struct OutputStreamMonitoringInfo {
  std::string stream_name;
  // The total number of packets added to the output stream.
  int num_packets_added;
  // The next timestamp bound of the output stream.
  Timestamp next_timestamp_bound;
};

mediapipe/mediapipe/framework/output_stream_handler.h

Lines 53 to 59 in eecf8a5

	struct OutputStreamMonitoringInfo {
	std::string stream_name;
	// The total number of packets added to the output stream.
	int num_packets_added;
	// The next timestamp bound of the output stream.
	Timestamp next_timestamp_bound;
	};