Make OIDC audience configurable

When trying to connect ESAP to the SKAO Rucio, this failed because the aud of the OIDC request is expected to be slightly different in this case. This is an extension of the original aud (adding an extra https://wlcg.cern.ch/jwt/v1/any to it, but I think it may be nicer to make this configurable using an environment variable.