[ENH] Allow for oidc usernames starting with a non alphabetic character

[Taeuber-hub]

Motivation: Allow for oidc usernames starting with a character that is not a-z or A-Z. This is caused by the "checkLoginNameValidity(String loginName) throws IllegalArgumentException" function in the UserAgentImpl.java class. This can be easily fixed by not throwing an exception in this case, since the username can be valid.

[lakhoune]

https://github.com/rwth-acis/las2peer/blob/f9ba175c377a4f6c63558d6f1fc45016b1013c9b/core/src/main/java/i5/las2peer/security/UserAgentImpl.java#L85