perf: Add ReflectionCache for Iceberg serialization optimization [iceberg]

[Shekharrajak]

Which issue does this PR close?

Closes #3456.

Rationale for this change

PR #3298 added reflection caching optimizations for Iceberg serialization, but these were lost during subsequent refactoring in #3349 and #3443. The current code performs redundant Class.forName() and getMethod() calls for every task (tens of thousands of times for large tables), causing significant serialization overhead.

What changes are included in this PR?

• Add ReflectionCache case class
• Update serializePartitions() to create cache once and pass to helper methods
• Update extractDeleteFilesList() and serializePartitionData() to use cached methods
• Add field ID mapping cache to avoid redundant buildFieldIdMapping() calls per-task
• Add CometIcebergSerializationBenchmark to measure serialization performance

How are these changes tested?

• Existing Iceberg integration tests ensure correctness is preserved

Benchmark:

Metric	Before	After	Improvement
serializePartitions()	7,235 ms	5,211 ms	28% faster
Class.forName()	233.5 ns	~0 ns	cached
getMethod()	18.2 ns	~0 ns	cached