Adds 'any' scalar, nullable optionals, wildcards to Picoschema.

[mbleigh]

Three quality-of-life improvements for Picoschema:

1. You can now specify any as a scalar type and it will allow anything.
2. When a field is marked as optional, Picoschema will also mark it as nullable. This fixes an annoying situation where LLMs return null values instead of omitting keys.
3. You can have a wildcard field in an object using (*) as a key.

Checklist (if applicable):

• Tested (manually, unit tested, etc.)
• Changelog updated
• Docs updated