Add support for mapping nested fields from the record value 

In some CDC use-cases (debezium this time) the incoming record (avro or json) is structured in this way:
```
value={after={myvalue=1, key=c4ca4238a0-1}, ...}
``` 
Currently it's not possible to do a mapping in this way
```
mapping: .., value=value.after.myvalue
```
The sink throws an error (I don't have the stacktrace anymore actually) saying the field value.after.myvalue doesn't exist