🔐 Check authentication proofPurpose on VC

[bdeneux]

Following this discussion, when a user tries to authenticate through a Verifiable Credential (VC), the validity check should verify if the purpose of the VC is for authentication.

To implement this feature, we need to ensure that the axoned binary can sign VCs with the authentication purpose. This feature depends on the implementation of the authentication purpose in the axoned binary, as tracked in axone-protocol/axoned#724.