Support Shadow plugin

[Goooler]

Shadow can work with maven-publish like:

plugins {
  java
  `maven-publish`
  id("com.gradleup.shadow")
}

publishing {
  publications {
    create<MavenPublication>("shadow") {
      from(components["shadow"])
    }
  }
  repositories {
    maven("https://repo.myorg.com")
  }
}

More configs and examples could be found at https://gradleup.com/shadow/publishing/.

It would be great to introduce native support in this plugin.

Note: com.gradle.plugin-publish has already patched Shadow, configs like

plugins {
  id("com.gradle.plugin-publish") version "latest"
  id("com.vanniktech.maven.publish") version "latest"
  id("com.gradleup.shadow")
}

could work perfect.

[Goooler]

I added a new flag to control the shadowed variant in GradleUp/shadow#1662.