Add ChunkSize option to DownloadStream API for Azure Files

[Copilot]

This PR adds ChunkSize as an input parameter to the DownloadStreamOptions for Azure Files, allowing clients to specify larger chunk sizes when downloading files. This addresses a performance issue that occurs when downloading large files.

Changes

• Added ChunkSize field to DownloadStreamOptions struct in models.go
• Added ChunkSize field to httpGetterInfo struct to store the value
• Updated DownloadStream method to pass the chunk size to the response
• Updated NewRetryReader method to use the specified chunk size
• Added tests to verify the functionality with different chunk sizes

The changes are minimal and maintain backward compatibility, as the ChunkSize field is optional with a default value that matches existing behavior.

Usage Example

// Set a custom chunk size for better performance with large files
resp, err := fileClient.DownloadStream(ctx, &file.DownloadStreamOptions{
    ChunkSize: 8 * 1024 * 1024, // Use 8MB chunks instead of default 4MB
})

Fixes #24650.

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