Allow disabling automatic user creation on OIDC authentication

When a user successfully authenticates via OIDC but does not already have a corresponding account on the XWiki instance, the authenticator currently creates that account automatically.

Some deployments only want known/pre-provisioned users to be able to log in. In that case, authentication should fail and no user account should be created.

Add a configuration option (wiki client configuration + oidc.createUser in xwiki.properties) defaulting to true for backward compatibility. When set to false:

• existing OIDC-linked users can still authenticate and be updated
• unknown users are rejected and no account is created