[Feat]: Add a new class with utility methods for creating artifacts

[fjuma]

Add a new class with utility methods for creating artifacts in https://github.com/a2aproject/a2a-java/tree/main/server-common/src/main/java/io/a2a/server/util.

We should add utility methods similar to the ones in the Python SDK:
https://github.com/a2aproject/a2a-python/blob/main/src/a2a/utils/artifact.py

Appropriate tests should be added too, e.g., use the Python tests as a guide:
https://github.com/a2aproject/a2a-python/blob/main/tests/utils/test_artifact.py

[leehuiseok]

@fjuma
Hi,I'd like to work on this issue.

I plan to add a new ArtifactUtils class under io.a2a.server.util with static methods for creating artifacts, similar to the Python SDK’s artifact.py (e.g., textArtifact, jsonArtifact).

I will also add corresponding unit tests in ArtifactUtilsTest, following the patterns from the Python tests (test_artifact.py).

Please let me know if this approach aligns with what you expect. Thanks!

[fjuma]

Your approach sounds great! I've assigned this to you, thanks!