fix: return refresh token expiration time on introspection

[mitar]

Related Issue or Design Document

Fixes #801.

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
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• I confirm that this pull request does not address a security vulnerability.
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Further comments

I kept common things shared but then made a special case for access tokens to add additional fields.