Add updateVersion gradle task from plugin template (#86)

See [template example here][1].

[1]: https://github.com/opensearch-project/opensearch-plugin-template-java/blob/3b805c245a3791003160dbd1a47762cc0d6cafc1/build.gradle#L117-L125

Signed-off-by: Andrew Ross <andrross@amazon.com>

build.gradle

@@ -200,3 +200,13 @@ tasks.withType(PublishToMavenRepository) {
         predicate.get()
     }
 }
+
+task updateVersion {
+    onlyIf { System.getProperty('newVersion') }
+    doLast {
+        ext.newVersion = System.getProperty('newVersion')
+        println "Setting version to ${newVersion}."
+        // String tokenization to support -SNAPSHOT
+        ant.replaceregexp(file:'build.gradle', match: '"opensearch.version", "\\d.*"', replace: '"opensearch.version", "' + newVersion.tokenize('-')[0] + '-SNAPSHOT"', flags:'g', byline:true)
+    }
+}