✨ feat: Add FileReader option for config file preprocessing

[vinicius73]

This PR adds a new FileReader option that allows users to preprocess configuration files before they are parsed by fig's decoders. This enables use cases like environment variable substitution, template processing, and custom file formats.

Use Cases

1. Environment Variable Substitution: Preprocess config files to expand environment variables
2. Template Processing: Apply custom transformations before parsing
3. Custom File Formats: Support encrypted or compressed configuration files
4. Dynamic Configuration: Modify configuration content based on runtime conditions

Example Usage

// Custom file reader that expands environment variables
fileReader := func(filePath string) (io.Reader, error) {
    file, err := os.Open(filePath)
    if err != nil {
        return nil, err
    }
    defer file.Close()
    
    content, err := io.ReadAll(file)
    if err != nil {
        return nil, err
    }
    
    // Expand environment variables in the content
    expanded := os.ExpandEnv(string(content))
    return strings.NewReader(expanded), nil
}

var cfg Config
err := fig.Load(&cfg, fig.WithFileReader(fileReader))