Revisiting ProxyConfiguration.getProxies()

[cowwoc]

@sbordet @joakime Revisiting #8723, I'm wondering whether it was wise/necessary to deprecate getProxies().

Maybe we could keep the getProxies() method but specify that it returns an unmodifiable List or Collection instead? This would restore the users' ability to look up the proxy list without exposing any implementation details. Users would still be forced to use the addProxy(), removeProxy() methods for mutation.