Add 'may-spotless-apply' profile (#611)

I often find myself forgetting to apply spotless before committing.

On some repositories, I have set up a pre-commit git hook running `mvn
spotless:check`. However, this has to be set up on each repository that
has spotless check enabled, and disabled on those which don't.

I think this goes one step further, by defining a conventional maven
profile `may-spotless-apply` that will run `spotless:apply` *only* if
spotless is enabled for the given repository.

This allows any user to incorporate `-Pmay-spotless-apply` as part of
their usual build command (or defined in their maven settings active profiles section),
and have `spotless:apply` automatically called if the project has it enabled.

pom.xml

@@ -1014,5 +1014,28 @@
         <yarn.lint.skip>true</yarn.lint.skip>
       </properties>
     </profile>
+    <profile>
+      <id>may-spotless-apply</id>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>com.diffplug.spotless</groupId>
+            <artifactId>spotless-maven-plugin</artifactId>
+            <executions>
+              <execution>
+                <id>spotless-apply</id>
+                <goals>
+                  <goal>apply</goal>
+                </goals>
+                <phase>validate</phase>
+                <configuration>
+                  <skip>${spotless.check.skip}</skip>
+                </configuration>
+              </execution>
+            </executions>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
   </profiles>
 </project>