build(core.gradle-plugin): Maven发布添加Plugin Marker构件

[shifujun]

便于插件工程以Gradle更新的plugins id语法引入插件。
plugins {
id 'com.tencent.shadow.plugin'
}

此PR为重提 #1319

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}