feat(csharp/Benchmarks): Add custom columns for CloudFetch benchmark metrics

[eric-wang-1990]

Summary

Add custom BenchmarkDotNet columns to display Peak Memory, Total Rows, and Total Batches in the summary table instead of requiring users to check console output.

Changes

• Add BenchmarkMetrics class to store peak memory, total rows, and total batches
• Store metrics in temp file (cloudfetch_benchmark_metrics.json) for cross-process access
• Update PeakMemoryColumn to read from temp file instead of static dictionary
• Add TotalRowsColumn to display total rows processed
• Add TotalBatchesColumn to display total batches processed
• Register all three custom columns in CloudFetchBenchmarkRunner
• Update README with .NET Framework 4.7.2 instructions for Power BI testing
• Update README with new metrics column documentation and examples

Problem Solved

This fixes the "See previous console output" issue where custom columns couldn't access metrics because BenchmarkDotNet runs iterations in separate processes. The temp file approach ensures metrics are available when generating the final summary table.

Before

| Peak Memory (MB)            |
|----------------------------:|
| See previous console output |

After

| Peak Memory (MB) | Total Rows | Total Batches |
|-----------------:|-----------:|--------------:|
| 256.48           | 1,441,548  | 145           |

Testing

• Built successfully on macOS with net8.0
• All three custom columns now display actual values in summary table
• Metrics written to temp file during execution
• README updated with net472 instructions for Windows/Power BI testing

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