feat(spec): Update A2A types from specification 🤖

[a2a-bot]

This PR updates src/a2a/types.py based on the latest specification/json/a2a.json from a2aproject/A2A.

[gemini-code-assist]

Summary of Changes

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This pull request incorporates updates from the latest A2A specification, primarily by introducing a new AgentCardSignature type and integrating it into the AgentCard structure. This enhancement enables the representation and handling of JSON Web Signatures for agent cards within the system's type definitions.

Highlights

• New Type Definition: Introduced a new AgentCardSignature class to represent JSON Web Signatures (JWS) for AgentCards. This class includes fields for protected (Base64url-encoded JWS header), signature (computed signature), and an optional header (unprotected JWS header values).
• AgentCard Enhancement: The AgentCard type has been updated to include a new optional field named signatures, which is a list of AgentCardSignature objects. This allows for the attachment of multiple JWS signatures to an AgentCard.
• Automated Specification Update: This pull request is an automated update, reflecting the latest changes from the A2A specification, ensuring that the local type definitions remain synchronized with the upstream source.

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[gemini-code-assist]

Code Review

This pull request updates the A2A types from the specification, adding the AgentCardSignature class and a signatures field to the AgentCard class. Reviewers should ensure that the new field is correctly handled throughout the codebase.