Add missing APIs, fix URL generation for create_file()

Add missing APIs for querying projects, files, approval rules and repository contributors

• group_projects(...)
• user_projects(...)
• get_file(...)
• approval_rules(...)
• repository_contributors(...)
• user_search(...)
• update_merge_request(...)
• delete_merge_request(...)
• push_rule(...)

Fix URL generation to escape file_path in create_file(...)