semantic-core: alternative version using cue

[rarguelloF]

This PR illustrates an alternative using Cue to generate the JSON Schema files instead of Pydantic.

It generates the following 2 schemas:

• agent_payload.json
• span.json

The span.json schema contains the logic of the existing intake_resolved_*_span.json schemas using the if/then/else JSON Schema feature: https://json-schema.org/understanding-json-schema/reference/conditionals#if-then-else

It also contains the changes made in #39