Create new BEST practice: device, app attestation with hardware backed keys

[Diolor]

The whole device, app integrity attestation with a server challenge with a hardware-backed key generation, is a best practice. The issues should be covered in the respective WEs and TESTS. However, we should combine all those aspects into a best practice.

Topics:

• Hardware-backed cryptographic keys (most likely asymmetric)
• App attestation
• Device attestation
• Attestation challenge

[cpholguera]

Perfect, thanks. Please assign it to yourself if you'd like to work on it.