Add Distinct cookbook example to Go SDK

Add the Go equivalent of

[https://github.com/apache/beam/blob/master/examples/java/src/main/java/org/apache/beam/examples/cookbook/DistinctExample.java](https://github.com/apache/beam/blob/master/examples/java/src/main/java/org/apache/beam/examples/cookbook/DistinctExample.java)

Imported from Jira [BEAM-3532](https://issues.apache.org/jira/browse/BEAM-3532). Original Jira may contain additional context.
Reported by: herohde.

Start by reading examples/java/src/main/java/org/apache/beam/examples/cookbook/DistinctExample.java, linked in the issue, to understand the intended Distinct cookbook example. Then locate the corresponding Go SDK cookbook examples and add the Go equivalent, verifying that the example provides the same behavior and is integrated consistently with nearby examples.

Written by the indexing model from the issue text.