spaceBeforeSeparator setter in JsonExtension

Hi, I was trying to set a custom value for spaceBeforeSeparator in Gradle Kotlin DSL, as shown in the Gradle plugin example for the Jackson formatted. However, the relevant property from JacksonJsonConfig was not exposed to JsonExtension.

spaceBeforeSeparator always false

After tweaking the plugin with a local snapshot to add the relevant setter, I noticed that the rule that I was trying to achieve (i.e. spaceBeforeSeparator=true) was not respected by spotless. The effective behavior was that expected with spaceBeforeSeparator=false. I digged a little bit more:

at com.diffplug.spotless.glue.json.JacksonJsonFormatterFunc$SpotlessJsonPrettyPrinter.withSeparators(JacksonJsonFormatterFunc.java:107)
at com.fasterxml.jackson.core.util.DefaultPrettyPrinter.<init>(DefaultPrettyPrinter.java:128)
at com.fasterxml.jackson.core.util.DefaultPrettyPrinter.<init>(DefaultPrettyPrinter.java:104)
at com.diffplug.spotless.glue.json.JacksonJsonFormatterFunc$SpotlessJsonPrettyPrinter.<init>(JacksonJsonFormatterFunc.java:92)
at com.diffplug.spotless.glue.json.JacksonJsonFormatterFunc$SpotlessJsonPrettyPrinter.createInstance(JacksonJsonFormatterFunc.java:100)
at com.diffplug.spotless.glue.json.JacksonJsonFormatterFunc$SpotlessJsonPrettyPrinter.createInstance(JacksonJsonFormatterFunc.java:88)
at com.fasterxml.jackson.databind.ObjectWriter$GeneratorSettings.initialize(ObjectWriter.java:1438)
at com.fasterxml.jackson.databind.ObjectWriter._configureGenerator(ObjectWriter.java:1312)
at com.fasterxml.jackson.databind.ObjectWriter.createGenerator(ObjectWriter.java:751)
at com.fasterxml.jackson.databind.ObjectWriter.writeValueAsString(ObjectWriter.java:1140)
...

SpotlessJsonPrettyPrinter#withSeparators(Separators) is responsible for tweaking _objectFieldValueSeparatorWithSpaces according to the user-provided value of spaceBeforeSeparator. However, this method is called by the default constructor of DefaultPrettyPrinter, before we have a chance to set the value of the field spaceBeforeSeparator. Thus, SpotlessJsonPrettyPrinter#spaceBeforeSeparator is always false when SpotlessJsonPrettyPrinter#withSeparators(Separators) is called.

PR content

In this PR, I address the following:

• The problem reported in the first paragraph (changes in JsonExtension);
• The problem reported in the remaining paragraphs (changes in JacksonJsonFormatterFunc).

Let me know if I'm missing something, I've tried the tweaked plugin and I observed the expected behavior.