Add option to skip issuer URL verification

[Vivida1]

The validator checks if the issuer URL and aud inside the access token match their corresponding provided configuration. It is possible to pass multiple audiences to the validator, but it is not possible to provide multiple issuer URLs. Without breaking and changing a lot of code, it seems the easiest way to allow for multiple applications (-> issuer URLs) was to just skip the issuer URL validation.

Afaik this does still comply with the specifications.

📝 Checklist

• [ x] All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

Where am I suppose to add the documentation?

🔧 Changes

A validator option called SkipIssuerURLVerification was added to skip the verification of issuer URLs in access tokens.

📚 References

#197
#276

🔬 Testing

See test in validator_test.go