[LG-324] Add piv/cac subject to attributes for move.mil

[jgsmith-usds]

Why:
Move.mil needs to know the subject of the piv/cac so
they can link the user's account to resources in DoD.

How:
Add the subject of the cert to the attribute bundle
provided via OIDC.

N.B.: Looks like I needed to tweak a few other things for the piv/cac subject to show up in a test OIDC client. Added them to this PR since they're all aimed at the same goal.

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[monfresh]

LGTM. Has the PIV/CAC service been updated to return the subject key?

[jgsmith-usds]

It was already using subject 🤦‍♂️ , so this is fixing the IdP to match (https://github.com/18F/identity-pki/blob/master/app/models/certificate.rb#L134).