get access token from EKM during setup

[tomholub]

The mechanism should be exactly the same as with access token from FES.

It will be a separate access token / stored separately, the two don't mix.

Related issue on the EKM https://github.com/FlowCrypt/flowcrypt-email-key-manager/issues/743

The EKM endpoint is GET /v1/access-token and it has the same structure and behavior as the existing FES endpoint.

[tomholub]

closing - EKM will remain working with OIDC only, without access token