move a2uitoolset to sdk

[wrenj]

move https://github.com/google/A2UI/blob/main/samples/agent/adk/rizzcharts/a2ui_toolset.py with its part converter to sdk here https://github.com/google/A2UI/blob/main/a2a_agents/python/a2ui_extension/src/a2ui/a2ui_extension.py.

it will need tests

also maybe add a eval script to test accuracy and quality of the LLM generated output since it does fail sometimes

I was thinking we could run fixers after generating the a2ui schema to correct common issues. One I noticed was the LLM putting a trailing "," after the last element in a list, which is noncompliant with JSON

[wrenj]

you can refactor to not depend on session state too, maybe a2ui_schema is json or callback<ToolContext, json> and enabled are bool | callback<ToolContext, bool>