user.name is always UUID when using OIDC and Spring Security

## Is your feature request related to a problem?

When using Spring Security oauth2 authentication to secure an API server, `user.name` in APM always shows the user ID (as it's the "name" of the Principal).

If a user describes an issue and I go into the APM service view to investigate, I don't typically have their UUID (and neither do they). I do however have their email (login) or preferred username, so I need to recover their UUID manually from Keycloak.

## Describe the solution you'd like

I'd like a different claim, in case of JWTs, to be logged instead of or in addition to the UUID. preferred_username is already mentioned in an Azure SSO fallback. Ideally, which claim is used would be configurable. Of course this is only valid for JWTs and similar tokens, which is my use case, so not entirely generic.

This may require going through the Spring MVC plugin instead of the Servlet one?

## Describe alternatives you've considered

Using MDC to get additional claims into the transaction data, but it doesn't fit well in the ECS schema and it requires adding code to every backend service.