Revert "Update Beam Protobuf Schema (Java) (#35150)"

[Abacn]

This reverts commit c3ef7f3.

Fix https://github.com/apache/beam/actions/workflows/beam_PreCommit_Yaml_Xlang_Direct.yml.

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[gemini-code-assist]

Summary of Changes

Hello @Abacn, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request reverts a recent update to the Beam Protobuf Schema in Java, aiming to resolve issues that were causing a specific GitHub Actions workflow to fail. The changes involve a significant internal refactoring of how Protobuf messages are translated to and from Beam Rows. This includes replacing the older ProtoBeamConverter with a new ProtoDynamicMessageSchema that employs a more granular, type-specific conversion strategy. Additionally, the handling of nullability for schema fields has been streamlined, leading to the removal of the FieldValueHaver interface and related ByteBuddy code. The Schema class's toString() method has also been updated to provide a more comprehensive output.

Highlights

• Revert of Protobuf Schema Update: This pull request primarily reverts a previous commit that updated the Beam Protobuf Schema in Java, specifically addressing a failing GitHub Actions workflow.
• Refactoring of Protobuf-to-Beam Conversion: The previous ProtoBeamConverter class has been entirely removed and its functionality replaced by a new, modular ProtoDynamicMessageSchema. This new implementation introduces a more structured approach with dedicated Convert classes for various Protobuf field types.
• Simplification of Nullability Handling: The explicit FieldValueHaver interface and its associated ByteBuddy generation logic for checking field presence have been removed. Nullability for Protobuf fields is now handled more directly within the schema translation and conversion logic, including updates to logical types to correctly handle null inputs.
• Enhanced Schema toString() Method: The Schema.java class now provides a more detailed and human-readable toString() representation, with this formatting logic moved directly into the Schema class and removed from SchemaUtils.

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