Add eclipse java code formatting as a (command line) gradle task

Our eclipse configured formatting of java code can be added as a gradle task using the gradle "spotless" plugin.

Since our formatting configuration for eclipse is already separated out into
.settings/org.eclipse.jdt.core.jalview.prefs
(so it can always be merged into a freshly made settings file by the gradle eclipse plugin) this formatting can be configured in build.gradle very straightforwardly:

plugin {
  id 'com.diffplug.gradle.spotless' version '3.24.2'
}
spotless {
  java {
    eclipse("4.12.0").configFile eclipse_extra_jdt_prefs_file
  }
}
// ("4.12.0" refers to the version number for Eclipse 2019-06)