Update EventConverter.java to populate metadata in io.a2a.spec.Part

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[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

## 🔴 Required Information
*Please ensure all items in this section are completed to allow for efficient
triaging. Requests without complete information may be rejected / deprioritized.
If an item is not applicable to you - please mark it as N/A*

### Is your feature request related to a specific problem?
Please describe the problem you are trying to solve. (Ex: "I'm always frustrated
when I have to manually handle X...")

The Response from LLM uses com.google.genai.types.Part and the https://github.com/google/adk-java/blob/main/a2a/src/main/java/com/google/adk/a2a/converters/EventConverter.java translates to io.a2a.spec.Part.

There is no way to set the metadata in the io.a2a.spec.Part from the com.google.genai.types.Part.

### Describe the Solution You'd Like
A clear and concise description of the feature or API change you want.
Be specific about input/outputs if this involves an API change.

Ability to add metadata in the Part will give us ability to segregate responses

### Impact on your work
How does this feature impact your work and what are you trying to achieve?
If this is critical for you, tell us if there is a timeline by when you need
this feature.
We do not want to deserialize the parts, to see what information it holds.

### Willingness to contribute
Are you interested in implementing this feature yourself or submitting a PR?
(Yes/No)
No

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## 🟡 Recommended Information

### Describe Alternatives You've Considered
A clear and concise description of any alternative solutions or workarounds
you've considered and why they didn't work for you.
Add the metadata section in the com.google.genai.types.Part and update `EventConverter.java` to set the same metadata

### Proposed API / Implementation
If you have ideas on how this should look in code, please share a
pseudo-code example.

### Additional Context
Add any other context or screenshots about the feature request here.