[FLINK-39044][benchmark] Add StringDataBenchmark for Table API string serialization

[nateab]

What is the purpose of the change

Add a JMH benchmark for StringDataSerializer operations to measure the performance of BinaryStringData copy, serialize, and deserialize operations.

This benchmark was used to validate the optimization in FLINK-39044 (apache/flink#27549), which adds a fast path in BinaryStringData.copy() for compact strings.

Benchmark methods:

• copyCompactString / copyLargeString - measures serializer.copy() performance
• serializeCompactString / serializeLargeString - measures serializer.serialize() performance
• deserializeCompactString / deserializeLargeString - measures serializer.deserialize() performance

Sample results showing FLINK-39044 optimization impact:

Benchmark	Baseline (ops/ms)	Optimized (ops/ms)	Improvement
copyCompactString	18,381 ± 22,770	807,198 ± 13,825	+4,292%
copyLargeString	3,551 ± 4,718	748,334 ± 6,211	+20,975%

Brief change log

• Added StringDataBenchmark.java in org.apache.flink.benchmark.full package

Verifying this change

This change added tests and can be verified as follows:

• Run the benchmark directly: mvn clean install exec:exec -Dbenchmarks=".*StringDataBenchmark.*"
• Or run via main method in the benchmark class

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable