Simplify version override for Dev versions of the Go SDK.

Right now, the Go SDK has a version constant that's ticked up every release cut at core.SDKVersion https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/core.go#L30

But it's presently a dev version for SDK developers, which means any containers or Jars that are requested, may not exist or be built. This is inconvenient for SDK developers.

The problem also extends to SDK release verification, as until the release is done, similarly no such versions exist in a published format. However, release validation should likely be validating against manually started Expansion services anyway, to ensure the version being tested is what's expected.

There should be a simple way to override the version string, in particular for Xlang transforms to simplify wrapper development.

Ultimately, it should probably be a function in the core package called by version aware code, and a variable that can be set by flag somewhere.

Imported from Jira [BEAM-13805](https://issues.apache.org/jira/browse/BEAM-13805). Original Jira may contain additional context.
Reported by: lostluck.