DefaultSigner should support key rotation

[mitar]

Preflight checklist

• I could not find a solution in the existing issues, docs, nor discussions.
• I agree to follow this project's Code of Conduct.
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Ory Network Project

No response

Describe your problem

It looks to me like there is no simple way to rotate the keypair? DefaultSigner and its GetPrivateKey allows only one key to be returned?

Describe your ideal solution

I think DefaultSigner should also have a method like GetRotatedPrivateKeys which would be used only for validating. Similar to how there is GlobalSecret and RotatedGlobalSecrets.

Workarounds or alternatives

I could implement my own Signer but it would be better if this is part of fosite.

Version

v0.45.0

Additional Context

No response