Support build agnostic external plugin SPI with dynamic lookups

[kwin]

Currently adding a custom plugin requires adaptations to the Maven/Gradle plugin as
a) FormatterStepFactory and
b) FormatterFactory or FormatExtension

are
a) not build system agnostic and
b) are not dynamically looked up

It would be beneficial to have a build-agnostic plugin SPI which allows to
a) load dependencies
b) pass configuration
c) dynamically lookup services from classpath (e.g. with https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/ServiceLoader.html)

For Maven the mechanism for adding additional classes to the plugin classpath is via plugin dependencies. I hope that Gradle provides something similar.

[nedtwigg]

For Gradle, you can just create a FormatterStep and add it.

spotless/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/FormatExtension.java

Lines 350 to 358 in b2a2cbb

	public void addStep(FormatterStep newStep) {
	requireNonNull(newStep);
	int existingIdx = getExistingStepIdx(newStep.getName());
	if (existingIdx != -1) {
	throw new GradleException(
	"Multiple steps with name '" + newStep.getName() + "' for spotless format '" + formatName() + "'");
	}
	steps.add(newStep);
	}

For Maven, the problem is the DSL. How do you configure which targets the new step applies to? Happy to take a design and then PR for a plugin SPI, but it should be maven only. I recommend writing the docs first.