Allow generating arbitrary (schemaless) JSON

[hudson-ai]

Calling guidance.json with an empty schema generates arbitrary JSON. This closes #887 -- to quote @wjn0, there are several motivations for this:

• APIs such as OpenAI allow users to request only valid JSON be generated sans schema, so in some sense this would give feature parity for local LLMs.
• Large JSON schemas often include "arbitrary" properties, e.g. properties that are allowed to be any valid JSON value: https://json-schema.org/understanding-json-schema/basics#hello-world!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.43%. Comparing base (14862a1) to head (84ce68c). Report is 2 commits behind head on main.

Files	Patch %	Lines
guidance/library/_json.py	93.33%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #892 +/- ## ========================================== + Coverage 56.80% 60.43% +3.63% ========================================== Files 63 64 +1 Lines 4625 4658 +33 ========================================== + Hits 2627 2815 +188 + Misses 1998 1843 -155 ```

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[hudson-ai]

@riedgar-ms ready for a final review when you have time.