Cast python dictionary into the betterpoto struct

Here the template as example:
```
message Test {
google.protobuf.Struct data = 1;
}
```
So, the idea is to add a function to cast a python dictionary in the protobuf structure. The value of dictionary can be any type: int, str, list, dict but we need to automatically set up the fields attribute for the `betterproto.lib.google.protobuf.Struct` instance.
Is this functionality exists? (for now, I just wrote a custom function for it)