Add checkstyleVersion Gradle CLI property

Related to https://github.com/checkstyle/checkstyle/issues/7306

According to discussion with Checkstyle team, we agreed to give us
project for testing their latest changes for regressions to avoid
situation like in the mentioned issue with `RequireThis`

* We expose a `checkstyleVersion` for Gradle command line to let to
provide a Checkstyle version externally.
This feature is going to be used on Checkstyle CI to run check in our
project against their SNAPSHOT

The command to run checks against external version is like this:
`./gradlew clean check --parallel -x test -PcheckstyleVersion=[CHECKSTYLE_VERSION]`
Where `[CHECKSTYLE_VERSION]` is a placeholder which should be replaced
with environment-specific variable or an explicit version.

build.gradle

@@ -330,7 +330,7 @@ subprojects { subproject ->
 
 	checkstyle {
 		configFile = file("$rootDir/src/checkstyle/checkstyle.xml")
-		toolVersion = '8.26'
+		toolVersion = project.hasProperty('checkstyleVersion') ? project.checkstyleVersion : '8.26'
 	}
 
 	artifacts {