[DRAFT] feat!: Upgrade A2A to v1.0

[Tehsmash]

Description

This updates the SDK to be A2A v1.0 compliant, all types are generated from the v1.0 a2a.proto. JSONRPC/HTTP+JSON transports are converted to use the a2a types encoded using ProtoJSON directly from the generated types.

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Fixes #559

[holtskinner]

This is a pretty good start on converting the types generation from the a2a.json to the a2a.proto. My main concern is that this is going to be a breaking change because the existing typing are based on Pydantic models, not Protobuf. This would need to be handled carefully to prevent breaking changes.