crypto: Add BitLocker Clear Key detection and passwordless open

[Marcondiro]

Hi,

This PR introduces support for Bitlocker Clear Keys: pre-provisioned and suspended BitLocker devices contain this on-disk key in clear, more info on Microsoft's FAQ.

In cryptsetup 2.8+, passing a NULL passphrase allows extracting the VMK using this Clear Key.

• Extend BDCryptoBITLKInfo with has_clearkey field
• In bd_crypto_bitlk_info(), probe for clear key presence
• In bd_crypto_bitlk_open_flags(), allow NULL context to trigger clear key activation
• Add test cases in tests/crypto_test.py

Assisted-by: Gemini

Thanks!

[coderabbitai]

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