Rename 'oidc_provider' Header (remove underscore)

[fxjordan]

The oidc_provider header contains an underscore. This seems to be allowed in HTTP, but so uncommon that Nginx needs a special configuration flag to allow for headers containing an underscore (https://dev.to/thesameeric/dont-use-underscores-in-your-http-headers-gfp).

In a recent bug, this caused a lot of extra work for me while find out why the OIDC provider was not set correctly. Sometimes, developers may even not know that a Nginx reverse proxy is running in front of their las2peer application and get strange behavior. Also, they might not be able to change the Nginx configuration in some cases.

Therefore, I'm proposing to rename the header to oidc-provider. The old header name can still be parsed for legacy reasons while we introduce the new header name.

https://github.com/rwth-acis/las2peer/blob/48084aa0952f5f105480714526d2e0b2fecef1ec/webconnector/src/main/java/i5/las2peer/connectors/webConnector/util/AuthenticationManager.java#L40