feat: add json mappings to default session and its contents

[sgal]

Related Issue or Design Document

This change adds json mappings to DefaultSession type and its contents to use in enriching the refresh webhook in Hydra here ory/hydra#3204

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.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got green light (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation within the code base (if appropriate).

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