Format Java source files automatically (#46745)

This commit adds a Java source formatter and checker into the build process.
This is not yet enabled for any sub-projects - to format and check a
sub-project, add its Gradle path into `build.gradle` and run:

    ./gradlew spotlessApply

to format, and:

    ./gradlew spotlessJavaCheck
    # or:
    ./gradlew precommit

to verify formatting.

Author: pugnascotia