Add API for setting task level properties from benchmark context

[ceresek]

Apparently, there is no elegant way of setting a task level property from benchmark context.

For setting a context level property, one can simply do:
ContextBuilder builder ...
builder.setProperty ("name", "value);

For setting a task level property, one probably has to do:
Task task ...
if (!task.isSetProperties ()) task.setProperties(new Properties ());
for (Property property : task.getProperties ().getProperty ())
if (property.isSetName () && name.equals (property.getName ())) {
property.setValue (value);
...
}
task.getProperties ().getProperty ().add (new Property ().withName (name).withValue (value));