Add support for side inputs/outputs to Java 8 lambdas in MapElements

Currently there's no way to use side inputs or outputs within Java 8 lambdas and MapElements. It would be nice if you could do something like this:

```

PCollection wordLengths = words.apply(
MapElements.via((String word) -> {
int sideInput1=
[[[ GetSideInputHere(); ]]]
[[[ SetSideOutputHere ]]] (sideInput1+word.length());
return
word.length();
}).withOutputType(new TypeDescriptor() {});

```

Imported from Jira [BEAM-656](https://issues.apache.org/jira/browse/BEAM-656). Original Jira may contain additional context.
Reported by: jpatokal.

Start with the MapElements API and its Java 8 lambda support, then consult the linked BEAM-656 Jira issue for context not included here. Done means Java 8 lambdas used with MapElements can access side inputs and produce side outputs while preserving the main output behavior.

Written by the indexing model from the issue text.