[Feat]: Update Part objects

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

Hello!
I am wondering if it's possible to update the Part interface, as well as the DataPart/FilePart/TextPart implementations in io.a2a.spec, in line with the [A2A specification of Part](https://a2a-protocol.org/latest/specification/#416-part)?

The main problem I am having with the current implementation is that I am having trouble doing JSON serialization/deserialization with the Part interface. Jackson/fastjson are unable to processes it.

### Describe the solution you'd like

1. Remove the Part interface, and DataPart/FilePart/TextPart classes.
2. Add a single Part class, with text, raw, url, data, metadata, filename, mediaType fields as described in the spec.

Thanks!

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

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