Streaming Demo Changes

[nichwch]

These changes are needed for the following demo to work: https://github.com/guardrails-ai/validator-hub-ui/pull/16

Main changes:

1. Fixes some bugs caused by adopting core schema impl interfaces.
2. Adds error_spans to output returned by stream validate requests. We still need to patch validators to include error_spans in FailResults, however, so the demo currently does not work.
3. Adds the Guard used in the demo to the config file.

[CalebCourier]

Just tested with the latest open source and existing client to server interactions are still working.
I think we can merge and release this as an alpha