fix: allow negative integers as duration in config

[littldr]

solves ory/hydra#2651

A temporary workaround until knadh/koanf#104 is merged. It allows us to use -1 as a valid value for a duration in the configuration of ory/hydra.

Related issue(s)

• Tokens expire after 30 days even if ttl.refresh_token=-1 hydra#2651

Checklist

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• 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
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• 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).

Further comments

[littldr]

this PR is superseded by #390