Validate function extension version

When I use the function API to create a function with withRuntimeVersion("beta"), the function could be created. But when I run the function, the function cannot work since its app setting variable `FUNCTIONS_EXTENSION_VERSION` set as `~beta` which is invalid. Should the library check this before creating the function?

```
FunctionApp function = azureClient.appServices().functionApps()
.define(testEnv.appServiceName)
.withRegion(Region.US_WEST)
.withExistingResourceGroup(testEnv.azureResourceGroup)
.withRuntimeVersion("beta")
.create();
```