[Java] Optimize beam pubsub message to serialized pubsub protobuf message

[scwhittle]

For the common case where just the payload is set, we can just copy directly to a byte array.
For the case where there are other set fields, we optimize protobuf construction by not calling methods on the builder for empty attribute map or empty strings.

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Summary of Changes

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This pull request significantly enhances the performance of serializing Beam PubsubMessage objects into Google Cloud Pub/Sub protobuf messages. It introduces optimizations to reduce overhead, particularly for messages consisting solely of a payload, by directly serializing the payload to a byte array. For messages with additional fields, it refines the protobuf builder usage to avoid setting empty or null values, contributing to more efficient message construction and overall improved serialization speed.

Highlights

• Serialization Optimization: Optimized the toProto method to avoid setting empty or null attributes, message IDs, and ordering keys on the protobuf builder, reducing unnecessary operations and improving efficiency.
• Direct Payload Serialization: Introduced a new private method toSerializedPubsubMessageProto for direct byte array serialization of PubsubMessage instances, specifically optimizing the common case where only the payload is present by using CodedOutputStream.
• Serializable Function Update: Updated the ParsePayloadAsPubsubMessageProto serializable function to leverage the new optimized toSerializedPubsubMessageProto path for improved performance.
• Comprehensive Testing: Added a new comprehensive unit test file, PubsubMessagesTest, to validate the correctness of toProto, fromProto, and the new optimized serialization logic across various message configurations, including cases with empty/null attributes, message IDs, and ordering keys.

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