Add new createProxy method on PrxHelper

[bernardnormier]

This PR adds a new createProxy static method on PrxHelper classes.

createProxy creates a proxy from a communicator and a proxy string. It's about simplifying this boilerplate code:

// old
using var communicator = Ice.Util.initialize(ref args);
var greeter = GreeterPrxHelper.uncheckedCast(
    communicator.stringToProxy("greeter:default -h localhost -p 10000"));

or

// old
using var communicator = Ice.Util.initialize(ref args);
var greeter = GreeterPrxHelper.checkedCast(
    communicator.stringToProxy("greeter:default -h localhost -p 10000"));

to:

// new, preferred
using var communicator = Ice.Util.initialize(ref args);
var greeter = GreeterPrxHelper.createProxy(
    communicator,
    "greeter:default -h localhost -p 10000");

I also updated the Ice/ami and Ice/operations tests to use this new syntax.

[externl]

Looks good!