add config module

[KARTIKrocks]

Added

• config — New config package: load application configuration from environment variables, .env files, and JSON config files into typed Go structs
• config — Load(dst, ...Option) populates a struct from sources in priority order (env vars > .env file > JSON file > default tags) and validates with request.ValidateStruct
• config — MustLoad(dst, ...Option) calls Load and panics on error, for use in main/init
• config — Struct tags: env:"VAR_NAME" maps fields to env vars, default:"value" sets fallbacks, validate:"..." reuses existing request validators
• config — Options: WithPrefix(p) for prefixed env vars (e.g., APP_PORT), WithEnvFile(path) for .env files, WithJSONFile(path) for JSON base config, WithRequired() to error on missing files
• config — Supported types: string, bool, int/int8/16/32/64, uint variants, float32/64, time.Duration, []string, []int
• config — Nested struct support: automatically flattened to env var names (e.g., DB.Host → DB_HOST)
• config — .env file parsing: comments, blank lines, quoted values (single/double), inline comments
• config — JSON config flattening: nested objects become underscore-separated uppercase keys
• config — Structured error messages using *errors.Error for parse errors, validation failures, and missing files