Renamed category variables to use underscores for better clarity and consistency. Updated client authentication method to accept username and password directly instead of using a Credentials struct.
Deleted client/credential.go and moved the Credentials struct to the api package for better modularity. Enhanced type safety in api by checking type assertions and added detailed comments in model structs.
This commit introduces new API and client modules that provide functionality for managing categories and products. It includes the creation of `.gitignore`, necessary data models, API endpoints, and client authentication using go-resty. This forms the foundation for further enhancements to the category and product management features.
