feat: treat default field in additionalProperties

[tomatommy-bs]

Reasons for making this change

feat #3915

Before this PR, adding a 'default' field to 'additionalProperty' did not affect the functionality. With this change, label/value will be added with the value specified in 'default' of 'additionalProperty'.

// sample schema

const schema = {
  type: "object",
  additionalProperties: {
    type: "string",
    default: "sample default"
  }
}

⚠️ Note

If a default value that does not conform to the schema is provided, the label/value will be added with that default value.

const schema = {
  type: "object",
  additionalProperties: {
    type: "string",
    default: 1
  }
}

In the above example, if an item is added, a label/value with the value 1 will be added.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown
• I'm adding or updating code
  • I've added and/or updated tests. I've run 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