feat(client): add context.Context support for cancellation and timeout

[xia0pin9]

This commit adds context.Context support throughout the knox client to enable proper cancellation and timeout handling for long-running operations.

Changes:

• Add context.Context parameter to cache and network key retrieval methods
• New methods: CacheGetKeyWithContext(), NetworkGetKeyWithContext(), getHTTPDataWithContext()
• Add context cancellation checks before expensive operations:
  • Before file I/O operations
  • Before JSON unmarshaling
  • Before each auth handler attempt
  • Before each retry attempt
  • Before backoff sleep
• Existing methods now delegate to *WithContext() variants with context.Background() for backward compatibility
• Fix bug in fileClient.setValues() where active keys were incorrectly appended instead of assigned to pre-allocated slice indices
• Standardize error logging (Knox→knox, %s→%w, .Error()→err)

The context support allows callers to:

• Set timeouts for key retrieval operations
• Cancel in-flight requests when they're no longer needed
• Properly handle deadline exceeded scenarios

All existing APIs maintain backward compatibility by using context.Background() internally.