New utility function features

Added new capability to the mergeDefaultWithFormData() utility function (see below)
Added new experimental_defaultFormStateBehavior.arrayMinItems.mergeExtraDefaults flag which caused a breaking change to the experimental arrayMinItems feature (see below):
- If you were previously passing experimental_defaultFormStateBehavior as { arrayMinItems = 'requiredOnly } on the Form, now you would pass { arrayMinItems: { populate: 'requiredOnly' } }

@rjsf/utils

Updated getDefaultFormState() to fix a bug where experimental_defaultFormStateBehavior: { emptyObjectFields: 'populateRequiredDefaults' } wasn't working for object properties with $refs
Experimental feature breaking change:
- Updated the experimental_defaultFormStateBehavior.arrayMinItems from simple flag to an object containing two optional fields, populate and mergeExtraDefaults
  - The new arrayMinItems.mergeExtraDefaults flag, when "true", allows users to merge defaults onto the end of formData arrays when minItems is specified
Added a new, optional mergeExtraArrayDefaults=false flag to the mergeDefaultWithFormData() utility function to support the new arrayMinItems.mergeExtraDefaults experimental feature

Updated the utility-functions documentation to add the new mergeExtraArrayDefaults flag for the mergeDefaultWithFormData() function
Updated the form-props documentation to update the arrayMinItems documentation for the new object behavior
Updated the playground to add a checkbox for the new arrayMinItems.mergeExtraDefaults flag