Implement skipIfExists for downloadBlobs in TransferManager

[nidhiii-27]

This PR implements the skipIfExists option for parallel downloads in TransferManager.

When skipIfExists is set to true in ParallelDownloadConfig, TransferManager.downloadBlobs will check if the destination file already exists on the local filesystem. If it does, the download for that blob will be skipped, and the result will have a status of SKIPPED.

Changes:

1. ParallelDownloadConfig.java: Added skipIfExists boolean and updated the Builder.
2. TransferManagerImpl.java: Added logic in downloadBlobs to perform the existence check and skip downloads.
3. DirectDownloadCallable.java: Added a safety check for skipIfExists before starting the download.
4. ParallelDownloadConfigTest.java: New unit tests for the configuration object.
5. TransferManagerTest.java: Added unit tests for the skip logic using Mockito.
6. ITTransferManagerTest.java: Added an integration test case to verify the skip behavior.

Unit tests passed successfully. Integration tests were added but verified manually due to environment-specific credential requirements.

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PR created automatically by Jules for task 6849503870578500544 started by @nidhiii-27

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