feat(spec): Add client-side protocol versioning in payloads

[lkawka]

Added protocol_version fields to request objects.

Fixes #1189

[gemini-code-assist]

Summary of Changes

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This pull request implements client-side protocol versioning by integrating a protocol_version field into key request and response payloads. This change is fundamental for managing API evolution and ensuring backward compatibility, allowing clients to explicitly declare the protocol version they are interacting with. The modifications span across gRPC, JSON schema, and TypeScript definitions to provide a consistent and robust versioning mechanism.

Highlights

• Client-Side Protocol Versioning: Introduced a new protocol_version field to various request and response objects, enabling client-side specification of the A2A protocol version.
• API Schema Updates: The protocol_version field has been added to the gRPC .proto definitions and the corresponding JSON schema (a2a.json) for consistency across API specifications.
• TypeScript Type Definitions: Updated TypeScript interfaces to include the optional protocolVersion property, ensuring type safety and developer clarity for client-side implementations.

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[gemini-code-assist]

Code Review

This pull request adds a protocol_version field to request payloads across gRPC, JSON, and TypeScript specifications to support client-side versioning. The changes are generally consistent. However, I've identified an inconsistency where protocol_version is added to a response object (ListTasksResponse) in the gRPC spec but not in its JSON or TypeScript counterparts. My review comment addresses this to ensure consistency across the API definitions.