Using ValueProvider<Long> with FixedWindows and Duration

Moving toward Apache Beam 2.x, I want to use templates as much as possible, and ValueProvider accordingly.

In my logic, I am using a FixedWindow, but the duration is flexible, so I rather get it from a ValueProvider.

The problem is that FixedWindows.of() only get Duration, and I failed to find a simple way to grab a ValueProvider and make a duration out of it.

According to Eugene Kirpichov "Unfortunately this question has a simple answer: this is currently not supported"

Imported from Jira [BEAM-3378](https://issues.apache.org/jira/browse/BEAM-3378). Original Jira may contain additional context.
Reported by: shushu.

Start with the Java SDK's FixedWindows.of() and ValueProvider APIs to understand where the duration is resolved. Define and test a supported path for template-provided durations, with completion shown by a FixedWindows use case that accepts ValueProvider.

Written by the indexing model from the issue text.