Add utility method to turn objects into dictionaries with custom keys

[hprange]

The ERXDictionaryUtilities.dictionaryFromObjectWithKeyMappings creates a dictionary from an object according to the given key mappings, skipping null values.

Usage:

NSDictionary<String, Object> data = dictionaryFromObjectWithKeyMappings(eo, NSDictionary.of("key1", EO.ATTRIBUTE1, "key2", EO.ATTRIBUTE2);

Result:

{
    "key1" = "valueForAttribute1";
    "key2" = "valueForAttribute2";
}