interface for mapping a value to a new value of arbitrary type.
This is a stateless record-by-record operation, i.e,
is invoked individually for each
record of a stream (cf.
for stateful value transformation).
is applied to a
record's key is preserved.
Note that the key is read-only and should not be modified, as this can lead to corrupt partitioning.
If a record's key and value should be modified
can be used.