Add builder for request construction

[nilsbehlen]

We often need to add another parameter to a request, so i think it is time to change the request construction to builder pattern. It should somewhat like this:
PIRequest req = privacyidea.requestBuilder().validateCheck(<required params>).transactionId(txid).clientip(clientip).build()
We can also offer more granularity by adding settings that are currently set on the privacyIDEA instance to just single requests (overwriting the global setting).

We can then also offer async or synchronous execution of the request in the PIRequest class.