feat: add ui:definitions for recursive and reusable uiSchema

[adamrimon]

Reasons for making this change

This PR adds a new ui:definitions property to uiSchema, enabling UI customizations for schema $ref references. This addresses a long-standing limitation where there was no way to define uiSchema for recursive schemas or to avoid repeating uiSchema for reused definitions.

Relates to #670 (Recursive references and uiSchema) and #477 (uiSchema for definitions)

The Problem:

// Schema with recursive definition
const schema = {
  definitions: {
    node: {
      type: 'object',
      properties: {
        name: { type: 'string' },
        children: { type: 'array', items: { $ref: '#/definitions/node' } }
      }
    }
  },
  properties: { tree: { $ref: '#/definitions/node' } }
};

// Previously: No way to define uiSchema for recursive nodes
// uiSchema only applied to the first level, not deeper recursion

The Solution:

const uiSchema = {
  'ui:definitions': {
    '#/definitions/node': {
      name: { 'ui:placeholder': 'Enter node name' }
    }
  }
};
// Now applies at ALL recursion levels

Key features:

• Define uiSchema once for schemas used in multiple places
• Support for recursive schemas at any depth
• Local overrides merge with and take precedence over definitions
• Different uiSchema for same property names in different oneOf/anyOf branches
• Keys use exact $ref paths (e.g., #/definitions/address or #/$defs/node)

Changes:

• @rjsf/utils: Added expandUiSchemaDefinitions() and resolveUiSchema() functions, UiSchemaDefinitions type, and related constants
• @rjsf/core: Integrated definition expansion at form initialization and runtime resolution in SchemaField
• @rjsf/playground: Updated References sample to demonstrate the feature
• @rjsf/docs: Added documentation for ui:definitions

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature