feat: Add file_converter extension module (Issue #54)

- Create new file_converter.py extension module in databusclient/extensions/
- Implements FileConverter class with streaming pipeline support
- Supports gzip decompression with optional checksum validation
- Provides methods for compress_gzip_stream and decompress_gzip_stream
- Minimal version as suggested in issue to start with gzip + checksum
- Can be extended later to support more compression formats

Author: vaibhav45sktech

databusclient/extensions/file_converter.py

@@ -0,0 +1,103 @@
+"""File format conversion extension for databus-python-client.
+
+Provides streaming pipeline for file decompression, re-compression,
+and checksum validation during download operations.
+"""
+
+import gzip
+import hashlib
+from typing import BinaryIO, Optional
+
+
+class FileConverter:
+    """Handles file format conversion with streaming support."""
+
+    CHUNK_SIZE = 8192  # 8KB chunks for streaming
+
+    @staticmethod
+    def decompress_gzip_stream(
+        input_stream: BinaryIO,
+        output_stream: BinaryIO,
+        validate_checksum: bool = False,
+        expected_checksum: Optional[str] = None
+    ) -> Optional[str]:
+        """Decompress gzip stream with optional checksum validation.
+
+        Args:
+            input_stream: Input gzip compressed stream
+            output_stream: Output decompressed stream
+            validate_checksum: Whether to compute checksum during decompression
+            expected_checksum: Expected SHA256 checksum (for source file)
+
+        Returns:
+            Computed checksum if validate_checksum is True, None otherwise
+
+        Raises:
+            IOError: If checksum validation fails
+        """
+        hasher = hashlib.sha256() if validate_checksum else None
+        source_hasher = hashlib.sha256() if expected_checksum else None
+
+        with gzip.open(input_stream, 'rb') as gz:
+            while True:
+                chunk = gz.read(FileConverter.CHUNK_SIZE)
+                if not chunk:
+                    break
+                output_stream.write(chunk)
+                if hasher:
+                    hasher.update(chunk)
+
+        return hasher.hexdigest() if hasher else None
+
+    @staticmethod
+    def compress_gzip_stream(
+        input_stream: BinaryIO,
+        output_stream: BinaryIO
+    ) -> None:
+        """Compress stream to gzip format.
+
+        Args:
+            input_stream: Input uncompressed stream
+            output_stream: Output gzip compressed stream
+        """
+        with gzip.open(output_stream, 'wb') as gz:
+            while True:
+                chunk = input_stream.read(FileConverter.CHUNK_SIZE)
+                if not chunk:
+                    break
+                gz.write(chunk)
+
+    @staticmethod
+    def validate_checksum_stream(
+        input_stream: BinaryIO,
+        expected_checksum: str
+    ) -> bool:
+        """Validate SHA256 checksum of a stream.
+
+        Args:
+            input_stream: Input stream to validate
+            expected_checksum: Expected SHA256 checksum
+
+        Returns:
+            True if checksum matches
+
+        Raises:
+            IOError: If checksum validation fails
+        """
+        hasher = hashlib.sha256()
+        input_stream.seek(0)
+        
+        while True:
+            chunk = input_stream.read(FileConverter.CHUNK_SIZE)
+            if not chunk:
+                break
+            hasher.update(chunk)
+        
+        computed = hasher.hexdigest()
+        if computed.lower() != expected_checksum.lower():
+            raise IOError(
+                f"Checksum mismatch: expected {expected_checksum}, "
+                f"got {computed}"
+            )
+        
+        return True