Add 'User-Agent' header to CLI REST calls

Update the InitializrService so that a 'SpringBootCli' User-Agent header
is sent with each request. This should allow the server-side code to
gracefully evolve the JSON format if needed.

Fixes gh-1869

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/init/InitializrService.java

@@ -141,6 +141,8 @@ class InitializrService {
 	private CloseableHttpResponse execute(HttpUriRequest request, Object url,
 			String description) {
 		try {
+			request.addHeader("User-Agent", "SpringBootCli/"
+					+ getClass().getPackage().getImplementationVersion());
 			return getHttp().execute(request);
 		}
 		catch (IOException ex) {