feat: add pooled serializer for Kafka exporter

[XiaoWeiKIN]

In opentelemetry-java, when using exporter span, object pools are supported to optimize memory usage. It is hoped that the same can be done when using kafka-exporter.

Description:

• Introduce PooledSpanDataSerializer with object pooling to reduce memory allocations
• Add MemoryMode support to KafkaSpanExporterBuilder
• Refactor SpanDataSerializer to use LowAllocationTraceRequestMarshaler
• Add comprehensive tests including thread safety validation

[linux-foundation-easycla]

• ❌ The email address for the commit (77133b4) is not linked to the GitHub account, preventing the EasyCLA check. Consult this Help Article and GitHub Help to resolve. (To view the commit's email address, add .patch at the end of this PR page's URL.) For further assistance with EasyCLA, please submit a support request ticket.