Previously the fields / classes are named with `placeholder`. However,
the actual purpose is to use as default values. This commit addresses
this problem by correcting names for relevant fields / classes.
- FormInputPlaceholder
- FormInput.placeholder
- HumanInputRequiredResponse.resolved_placeholder_values
- HumanInputFormDefinition.resolved_placeholder_values
- FormCreateParams.resolved_placeholder_values
- HumanInputRequired.resolved_placeholder_values
- The `resolved_placeholder_values` argument of _create_human_input_delivery_test_form
- The `resolved_placeholder_values` inside _jsonify_form_definition