feat(jans-cedarling): relax JwtService validation to allow optional claims

[rmarinn]

Is your feature request related to a problem? Please describe.

The current validation in JwtService is overly strict, requiring fields that may not be present in all token scenarios. This can prevent certain tokens from being accepted even when they are otherwise valid for Cedarling's purposes. We need more flexibility to allow for tokens that might be missing some non-essential claims.

Describe the solution you'd like

• iss should be OPTIONAL
• aud should be OPTIONAL
• sub should be OPTIONAL
• exp should be OPTIONAL but check validity if there is one
• nbf should be OPTIONAL but check validity if there is one