Bring study create up to date with API spec

[SeanAlexanderHarris]

The prolific study create command has fallen out of sync with the current API spec.

Objective

Update the CreateStudy model and related structs to align with the current Prolific API specification, replacing deprecated fields and adding new required and
optional fields.

Critical Changes

Breaking Changes (with Backward Compatibility)

• Replace completion_code (string) with completion_codes (array of objects)
• Replace completion_option (string) - deprecated
• Replace eligibility_requirements with filters or filter_set_id
• Replace access_details_collection_id with access_details array for taskflow studies

New Required Fields

• completion_codes - array of CompletionCode objects with code, code_type, actions fields

New Optional Fields

• access_details - array for taskflow studies with multiple URLs
• filter_set_id, filter_set_version - for using predefined filter sets
• is_custom_screening - custom screening flag
• content_warnings, content_warning_details - content warnings
• metadata - custom metadata as map[string]interface{}
• is_external_study_url_secure - security flag for JWT signing
• auto_rejection_categories - add to SubmissionsConfig struct