Support passing python types to scalars in casts. (#953)

Instead of generating cast functions for everything and letting the server figure it out, generate cast functions for scalars based on the reflected explicit casts.

Add support to convert python types into gel scalars if possible. For example:
```py
std.str.cast(1)
std.str.cast("abc")
```

This works for basic scalars (like `int64` and `str`) as well as enums.