Introduce WritableFS

[olekukonko]

The current Extraction functions make an assumption that writers will always write to a local directory. This limits flexibility and prevents integration with various storage backends like:

• Block storage systems
• HTTP APIs
• In-memory storage
• Temporary file systems
• etc

Proposed Solution:
Introduce a WritableFS interface that abstracts file system operations, allowing for multiple storage implementations. This provides:

// WritableFS defines an interface for a writable file system or directory-like storage.
type WritableFS interface {
    // Base returns the base path or identifier of the file system (e.g., directory path or "memory").
    Base() string

    // WriteFile writes data to the named file, creating it if necessary.
    // If the file exists, it may overwrite it (implementation-dependent).
    // The perm parameter specifies the file permissions (e.g., 0644).
    WriteFile(name string, data []byte, perm fs.FileMode) error

    // Mkdir creates a directory named path, along with any necessary parents.
    // The perm parameter specifies the directory permissions (e.g., 0755).
    Mkdir(path string, perm fs.FileMode) error
}

1. Better separation of concerns
2. Flexibility to use different storage backends
3. Cleaner API surface
4. Improved testability with in-memory implementations

Key Changes:

1. Added WritableFS
2. Implemented Two concrete types:
   • OsFS for local file system operations
   • MemFS for in-memory storage (useful for testing)

Example Usage:

// Using local filesystem
osFs := NewOsFS("/output/dir")

// Using in-memory storage (for testing)
memFs := NewMemFS()

// Normal way:
ExtractContent(inFile, "/output/dir", pages, conf)

// Using WritableFS:
ExtractContentFS(inFile, NewOsFS("/output/dir"), pages, conf)

Benefits:

• Backward compatible (can still use local file system)
• Enables new storage integrations without changing core logic
• Makes code more testable
• Follows Go's interface-oriented design principles
• Opens possibilities for future storage implementations (S3, GCS, etc.)

This is a production code hence Baby steps are required, hence i wanted to start with ExtractContentFS

[CLAassistant]

All committers have signed the CLA.

[hhrutter]

Please create a feature request issue.
PRs are the result of issue discussions.
Thank you!