Gracefully handle encoding of different number types

If a float is set on a Message field of type int then an exception is raised when serialising. Floats and ints are largely interchangeable in python3, so maybe betterproto should handle this more gracefully, by for example: casting to int or float as appropriate before encoding.

The resulting error message includes:

```
unsupported operand type(s) for &: 'float' and 'int'
```

Reproduce the issue by assigning a float to an int-typed Message field and serialising it, then inspect the encoding path that raises the reported operand-type error. The payload names no files or tests; done means the value serialises without that exception and follows the intended numeric representation.

Written by the indexing model from the issue text.