Update ExternalJavaProvider.available method to support Windows systems

[eug]

The current implementation of the ExternalJavaProvider.available method only supports Unix/Linux systems. This issue aims to update the function to also support Windows systems.

Changes:

The available method will be updated to use the where command on Windows systems, instead of the which command used on Unix/Linux systems.

New implementation:

class ExternalJavaProvider(ExternalProvider):
  def __init__(self, urns, jar_provider):
    super().__init__(
        urns, lambda: external.JavaJarExpansionService(jar_provider()))
    self._jar_provider = jar_provider

  def available(self):
      cmd = 'where' if sys.platform == 'win32' else 'which'
      return subprocess.run([cmd, 'java'], capture_output=True).returncode == 0