Enable device attestation JWT handler

[Haishi2016]

Enable using Azure Attestation Service JWT Token. This would need a bootstrapping step where attestation certificate public keys are acquired. This can be done by the Settings vendor, who's responsible for setting up running environment for Symphony components. When the vendor is loaded, it can perform handshake with Azure Attestation Service to get the keys.

[msftcoderdjw]

Hi @Haishi2016 , does this issue mean we need to integrate JWT token validation with AAD?

[Haishi2016]

Validation with AAD already works