Return actual enum's from to_dict

Perhaps I don't understand the intended use of the `to_dict` method but, at least for my use case, I would prefer it to return the actual enum instead of it's name as a string.

For example:

```
class EnumResultGeneric(betterproto.Enum):
RESULT_UNKNOWN = 0
RESULT_SUCCESS = 1

@dataclass
class ResponseGeneric(betterproto.Message):
result: "EnumResultGeneric" = betterproto.enum_field(1)

status = ResponseGeneric.FromString(bytes(bytearray([0x08, 0x01]))).to_dict()['result']
print(f"{status}: {type(status)}")
```

gives `RESULT_SUCCESS: ` instead of the actual `EnumResultGeneric`.

It is a simple change to get this desired behavior in the `to_dict` method:

```
elif meta.proto_type == TYPE_ENUM:
enum_values = list(
self._betterproto.cls_by_field[field.name]
) # type: ignore
if isinstance(v, list):
output[cased_name] = [enum_values[e] for e in v]
else:
output[cased_name] = enum_values[v]
```

Is this desirable for other people? Maybe allow this as a parameter to `to_dict`?