public Optional<String> getProxyUsername() {
        return Optional.ofNullable(proxyUsername);
    }
Returns the optional proxy user name to use if connecting
through a proxy.  Returns either the proxyUsername set on this
object, or if not provided, checks the value of the Java system
property for proxy user name according to {@link #getProtocol()}:
i.e. if protocol is https, returns the value of the system
property https.proxyUsername, otherwise returns value of
http.proxyUsername.

@return The optional proxy user name the configured client will use if connecting through a
        proxy.
aws-java-sdk-opensdk/src/main/java/com/amazonaws/opensdk/config/ProxyConfiguration.java