fix(core/auth/oauth): Properly mark OIDC callback error handler as optional

[carstendietrich]

By mistake, the newly introduced interface was not properly declared as optional but rather looked for an instance that was annotated with the name optional.

During testing, I also realized that it would make sense to have the original return URL within the function.
Since this interface was introduced with the release yesterday and was not working at all I'll flag this only as a fix.

[github-actions]

Package	Line Rate	Health
flamingo.me/flamingo/v3/core/auth	13%	❌
flamingo.me/flamingo/v3/core/auth/fake	50%	➖
flamingo.me/flamingo/v3/core/auth/http	40%	❌
flamingo.me/flamingo/v3/core/auth/oauth	48%	❌
flamingo.me/flamingo/v3/core/cache	54%	➖
flamingo.me/flamingo/v3/core/gotemplate	87%	✔
flamingo.me/flamingo/v3/core/healthcheck/interfaces/controllers	76%	✔
flamingo.me/flamingo/v3/core/healthcheck	64%	➖
flamingo.me/flamingo/v3/core/internalauth/application	100%	✔
flamingo.me/flamingo/v3/core/internalauth	100%	✔
flamingo.me/flamingo/v3/core/locale/application	87%	✔
flamingo.me/flamingo/v3/core/locale/domain	75%	✔
flamingo.me/flamingo/v3/core/locale/interfaces/controllers	100%	✔
flamingo.me/flamingo/v3/core/locale/interfaces/templatefunctions	92%	✔
flamingo.me/flamingo/v3/core/locale	59%	➖
flamingo.me/flamingo/v3/core/oauth/application	4%	❌
flamingo.me/flamingo/v3/core/oauth/domain	62%	➖
flamingo.me/flamingo/v3/core/oauth	55%	➖
flamingo.me/flamingo/v3/core/requestlogger	100%	✔
flamingo.me/flamingo/v3/core/requesttask	10%	❌
flamingo.me/flamingo/v3/core/robotstxt	50%	➖
flamingo.me/flamingo/v3/core/runtime	77%	✔
flamingo.me/flamingo/v3/core/security/application/role	95%	✔
flamingo.me/flamingo/v3/core/security/application	98%	✔
flamingo.me/flamingo/v3/core/security/application/voter	100%	✔
flamingo.me/flamingo/v3/core/security/interface/controller	100%	✔
flamingo.me/flamingo/v3/core/security/interface/middleware	72%	➖
flamingo.me/flamingo/v3/core/security	59%	➖
flamingo.me/flamingo/v3/core/zap	43%	❌
flamingo.me/flamingo/v3/framework/cmd	12%	❌
flamingo.me/flamingo/v3/framework/config	49%	❌
flamingo.me/flamingo/v3/framework/flamingo	30%	❌
flamingo.me/flamingo/v3/framework	29%	❌
flamingo.me/flamingo/v3/framework/opencensus	88%	✔
flamingo.me/flamingo/v3/framework/prefixrouter	43%	❌
flamingo.me/flamingo/v3/framework/systemendpoint	27%	❌
flamingo.me/flamingo/v3/framework/testutil	0%	❌
flamingo.me/flamingo/v3/framework/web	49%	❌
Summary	49% (3278 / 6678)	❌