PascalCase support

For a PascalCase field, betterproto uses the snake case in the class field, and can handle multiple cases in input json. However, it converts it to the camel case in output json, while protobuf requires a pascal case in the input. This makes the json incompatible.

For example,

```protobuf
message Test {
int32 camelCase = 1;
my_enum snake_case = 2;
snake_case_message snake_case_message = 3;
int32 PascalCase =4;
}
```

betterproto will output
```json
{"camelCase": 1, "pascalCase": 3, "snakeCase": "ONE"}
```
but protobuf is expecting
```json
{"PascalCase": 3, "camelCase": 1, "snakeCase": "ONE"}
```