Add foojay-resolver-convention plugin (#905)

See https://github.com/gradle/foojay-toolchains. With this plugin, if
relevant toolchains are not present on the local machine (e.g., for JDK
8 or JDK 21), Gradle will automatically download an appropriate
distribution and place it in the local Gradle cache. This plugin is
officially supported by Gradle, so I think it's safe to add. Without
this plugin, new contributors may run into issues running basic commands
like `./gradlew compileJava` since it requires multiple JDKs to be
installed in order to succeed.

settings.gradle

@@ -6,6 +6,11 @@ pluginManagement {
     }
 }
 
+plugins {
+    // to automatically pull in relevant JDK toolchains when not present on the local machine
+    id("org.gradle.toolchains.foojay-resolver-convention") version "0.8.0"
+}
+
 include ':annotations'
 include ':nullaway'
 include ':sample-library-model'