There should be a way to compose generators with filters

Maybe I'm missing something but something like this should be builtin:

```python

def COMPOSE(generator: ServiceValue, *callables):
class Generator(ServiceValue):
def gen_value(self, type_mixer, name, field):
field, value = generator.gen_value(type_mixer, name, field)
for func in callables:
value = func(value)
return field, value
return Generator()

```

eg: `mixer.blend(MyModel, field=COMPOSE(mixer.RANDOM, str.upper))`

Start by locating ServiceValue and its gen_value method, then trace how mixer.blend applies field values. Verify the proposed composition with a generator and callable such as mixer.RANDOM and str.upper; done means generator output can be passed through one or more filters.

Written by the indexing model from the issue text.