Support specifying a download location for the SDK

[rohanliston]

There doesn't appear to be a way to specify where the SDK will be downloaded to, which makes the SDK files more difficult to cache in a CI environment and prevent them from being downloaded/installed every time.

There is tools.cloudSdkHome, but the documentation for that property indicates that the download will only occur if this property is unset:

tools.cloudSdkHome: Optional property to configure the location of the Cloud SDK.
If this property is not specified, then the plugin automatically downloads Cloud SDK.

It would be great to perhaps have a secondary property to configure whether or not the SDK is downloaded to the specified path, e.g.

tools.cloudSdk.home=/path/to/sdk
tools.cloudSdk.downloadIfMissing=true