[Feat]: Add `Task` progress

### Is your feature request related to a problem? Please describe.

Right now there's no standard way of communicating `Task` progress with A2A, resulting in a bad user experience.

I have built typical, conversational agents with A2A. They accept text, have a number of tools they can call, return the results of the tool calls as `Artifact`s, and send intermediate messages the agent generates as `TaskStatusUpdateEvent`s.

However, because there's no standard way of communicating `Task` progress, my only option is to send `TaskStatusUpdateEvent`s.

The problem with `TaskStatusUpdateEvent`s is that:
1. They would interfere with the intermediate messages the agent generates. The first message might be "Sure, let me help you with that.", followed by "Tool Call X 1/100", "Tool Call X 2/100", "Tool Call X 3/100", ..., and finally "Here are the results I found".
2. Because they're typically text, there's no way of displaying them in a UI. I understand data could be sent, but without a standard the format will vary agent to agent.

This pattern is common enough that a standard would make sense.

### Describe the solution you'd like

Something like MCP Progress: https://modelcontextprotocol.io/specification/2025-03-26/basic/utilities/progress.

Because a `Task` can have multiple tool calls it might make sense to have a unique ID associated with each progress. An ideal solution would allow updates from multiple unique IDs at once for parallel tool calls.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

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