Add Go Autocomplete example

Add Go version of autocomplete example:

https://github.com/apache/beam/blob/master/examples/java/src/main/java/org/apache/beam/examples/complete/AutoComplete.java

There is an old stub present under examples/complete/autocomplete that can be repurposed.

Imported from Jira [BEAM-4467](https://issues.apache.org/jira/browse/BEAM-4467). Original Jira may contain additional context.
Reported by: herohde.

Start by reading examples/java/src/main/java/org/apache/beam/examples/complete/AutoComplete.java and the existing stub under examples/complete/autocomplete. Port the referenced autocomplete example into Go, preserving its behavior and example placement; done means the Go example is complete and usable alongside the Java version.

Written by the indexing model from the issue text.