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

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🔴 Required Information

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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

🟡 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

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pseudo-code example.

Additional Context

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