Include necessary metadata in gradle plugin POM

[maths22]

This should fix the publication error when pushing to gradle by adding the relevant metadata to the POM. This will create a plugin marker on Maven central, which should allow plugin configuration through the plugin DSL.

In settings.gradle:

pluginManagement {
    repositories {
        mavenCentral()/mavenLocal()/maven {
            url "https://oss.sonatype.org/content/repositories/snapshots"
        }
        gradlePluginPortal()
    }
}

In build.gradle:

plugins {
    ...
    id 'io.github.fvarrui.javapackager.plugin' version '1.7.3-SNAPSHOT'
}

[fvarrui]

Hi @maths22! I'll give you feedback when releasing 1.7.3. Thanks