[FEATURE] : Add synchronized buffer plugin

[MohammedAghil]

Add a synchronized buffer implementation that provides synchronous data flow from source to sink while leveraging parallel processing via Data Prepper's process worker threads. Writers block until their records are fully processed and checkpointed by worker threads.

Description

In the existing zero buffer, the caller thread handles everything — writing, processing, and publishing to sink — which means process worker threads are unused. The synchronized buffer addresses this by allowing the caller thread to write a batch of records into a shared, thread-safe queue and then block until all records in that batch have been fully processed and checkpointed by the worker threads. This enables use cases like the OpenSearch API source where the HTTP response must only be returned after the data has been delivered to the sink.

Key design:

• The caller wraps records in a SignaledBatch and enqueues it, then blocks waiting for completion
• Multiple process worker threads read slices from the batch concurrently, respecting a configurable batch_size
• On checkpoint, each worker marks its slice as processed; once all records in the batch are accounted for, the blocked caller is unblocked
• Metrics are emitted for records written and records read

Issues Resolved

Resolves #5712

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
• New functionality has javadoc added
• Commits are signed with a real name per the DCO

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[github-actions]

⚠️ License Header Violations Found

The following newly added files are missing required license headers:

• data-prepper-plugins/synchronized-buffer/build.gradle
• data-prepper-plugins/synchronized-buffer/src/main/java/org/opensearch/dataprepper/plugins/buffer/SynchronizedBuffer.java
• data-prepper-plugins/synchronized-buffer/src/main/java/org/opensearch/dataprepper/plugins/buffer/SynchronizedBufferConfig.java
• data-prepper-plugins/synchronized-buffer/src/main/java/org/opensearch/dataprepper/plugins/buffer/model/ReadBatch.java
• data-prepper-plugins/synchronized-buffer/src/main/java/org/opensearch/dataprepper/plugins/buffer/model/SignaledBatch.java
• data-prepper-plugins/synchronized-buffer/src/test/java/org/opensearch/dataprepper/plugins/buffer/SynchronizedBufferTests.java

Please add the appropriate license header to each file and push your changes.

See the license header requirements: https://github.com/opensearch-project/data-prepper/blob/main/CONTRIBUTING.md#license-headers