Support ktfmt dropbox style in maven plugin

[theminecoder]

This option exists in the Gradle plugin but currently does not in the maven plugin. This exposes it in a similar way to how the Gradle plugin does.

[nedtwigg]

Thanks! The GoogleJavaFormat maven step handles this situation like this:

spotless/plugin-maven/src/main/java/com/diffplug/spotless/maven/java/GoogleJavaFormat.java

Lines 29 to 30 in 4002aff

	@Parameter
	private String style;

The gradle plugin for GJF handles it like this:

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

Lines 100 to 107 in 4002aff

	public void style(String style) {
	this.style = Objects.requireNonNull(style);
	replaceStep(createStep());
	}
	public void aosp() {
	style("AOSP");
	}

And the gradle plugin for ktfmt handles it like this:

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

Lines 111 to 114 in 4002aff

	public void dropboxStyle() {
	style = Style.DROPBOX;
	replaceStep(createStep());
	}

Based on these precedents, what do you think about switching to a String style?

[theminecoder]

The problem is that the ktfmt step expects an enum value rather then a string so we will need to convert and validate the options users can provide.

Also I was trying to keep to exposure of this option the same as gradle so if I was to make this into a string I would probably end up updating gradle to take a string (and run it through the validation)

So if you are happy for me to do that I can :)

[nedtwigg]

Shipped in plugin-maven 2.7.0.