Add JMH benchmark infrastructure

Prior to this commit, the Spring Framework test suite would rely only on
"Performance" tests associated with a specific CI build. As outlined in
gh-24830, the way they're built and executed is not working well
anymore.

This commit introduces a new JMH benchmark infrastructure in the build.
The goal here is not to run those benchmarks as part of a CI build, but
rather provide a proper infrastructure for writing and locally running
micro-benchmarks when working on specific optimizations.

This commit adds and configures a Gradle JMH plugin to allow for JMH
benchmark classes in Spring Framework modules (in `src/jmh/java` of each
`spring-*` module). It's also relaxing the checkstyle rules for JMH
classes, especially around Javadoc rules: this code is not meant to
have Javadocs.

Finally, this commit links to a new Wiki page on the project GitHub
repository documenting the infrastructure and helping contributors to
run and design benchmarks.

See gh-24830

src/checkstyle/checkstyle-suppressions.xml

@@ -7,6 +7,9 @@
 	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/]java[\\/]org[\\/]springframework[\\/].+(Tests|Suite)" checks="IllegalImport" id="bannedJUnitJupiterImports" />
 	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/]java[\\/]" checks="SpringJUnit5" message="should not be public" />
 
+	<!-- JMH benchmarks -->
+	<suppress files="[\\/]src[\\/]jmh[\\/]java[\\/]org[\\/]springframework[\\/]" checks="JavadocVariable|JavadocStyle" />
+
 	<!-- spring-beans -->
 	<suppress files="TypeMismatchException" checks="MutableException"/>
 	<suppress files="BeanCreationException" checks="MutableException"/>