chore(java): spilt core and examples & support both Spring AI and ADK

[AlbumenJ]

Description

• Split core and examples. Now users can using java sdk seperately
• Update model to A2A v0.2.1
• Support both Spring AI and ADK as a demo server
• Successfully integrate with python hello-world and cli

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Fixes #<issue_number_goes_here> 🦕

[AlbumenJ]

@holtskinner PTAL. Thanks

[holtskinner]

@pstephengoogle Can you review? Should we turn these classes into the Java SDK?

[rajeshvelicheti]

@AlbumenJ thank you so much for this contribution. I had a chance to review this and I think the idea might be very useful. We might want to target a java sdk similar to the python sdk. Primarily, the idea is to follow a similar design pattern, so we could support multiple transports as we progress.

I am happy to collaborate with you further on this.

[AlbumenJ]

@AlbumenJ thank you so much for this contribution. I had a chance to review this and I think the idea might be very useful. We might want to target a java sdk similar to the python sdk. Primarily, the idea is to follow a similar design pattern, so we could support multiple transports as we progress.

I am happy to collaborate with you further on this.

@rajeshvelicheti Thanks for your kindly review. I will update the code later.

[AlbumenJ]

@rajeshvelicheti I have updated all the code according to your suggestion. Please review again.

[holtskinner]

We now have a published java sdk https://github.com/a2aproject/a2a-java

Can you convert these samples to use the new SDK?