Create a CLI to convert a pipeline to graphviz dot format

As a Beam developer (especially one using python or go) I want a way to convert a pipeline to graphviz dot format so that I can easily visualize it.  

The Beam source comes with a great utility in the form of [PortablePipelineDotRenderer.java](https://github.com/apache/beam/blob/2df702a1448fa6cbd22cd225bf16e9ffc4c82595/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/renderer/PortablePipelineDotRenderer.java#L29). It seems like a great util to wrap with a CLI, so that it can be easily accessed from other languages and just for general ease-of-use for java developers.

The tool would take an input pipeline message as either a path to a file or via stdin, and would output a dot string to stdout or to a file.

I'm happy to make a PR for this.

Imported from Jira [BEAM-7677](https://issues.apache.org/jira/browse/BEAM-7677). Original Jira may contain additional context.
Reported by: chadrik.