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
This ensures that `HumanInputForm` could be associated to a specific
application without relying on `WorkflowRun`, providing us a smoother
migration path if we want to implement test form.