Finalize A2UI template schema and data-binding syntax

Establish the formal technical contract for how templates are stored and validated to ensure interoperability between the Composer and the Agent SDK.

While https://github.com/google/A2UI/pull/365/ provides an initial [A2UI_COMPONENT_CATALOG](https://github.com/maxkorp/A2UI/blob/72fd136002d24aa786e773744236fad3283e0bb1/a2ui-composer/src/app/api/copilotkit/a2ui-prompt.ts#L4-L39), we need a reusable specification for the Template.

The schema can be stored in the `template` sub-package of the Agent SDK.

Proposed tasks:
- [ ] Define the template schema in the Agent SDK
- [ ] Formalize the syntax for dynamic placeholders using `{{}}`
- [ ] Document the template schema to standardize how the LLM returns selected template names and populated data
- [ ] Provide a catalog of 3-5 reference templates as examples.