Update Gradle Build to include license info in artifact POM

[hpmellema]

Issue #, if available: Closes #59

Description of changes:
This pull request adds licensing and Source control info to the published POM artifact.

It also handles an error where the mavenJava publication was overwritting the built-in pluginMaven publication. The updated version here extends the existing pluginMaven publication rather than creating a new publication.
For more details see this issue: gradle/gradle#10384

Testing
The following is the generated POM in my MavenLocal repo with the changes:

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <!-- This module was also published with a richer model, Gradle metadata,  -->
  <!-- which should be used instead. Do not delete the following line which  -->
  <!-- is to indicate to Gradle or any Gradle module metadata file consumer  -->
  <!-- that they should prefer consuming it instead. -->
  <!-- do_not_remove: published-with-gradle-metadata -->
  <modelVersion>4.0.0</modelVersion>
  <groupId>software.amazon.smithy</groupId>
  <artifactId>smithy-gradle-plugin</artifactId>
  <version>0.6.0</version>
  <description>This project integrates Smithy with Gradle. This plugin can build artifacts from Smithy models, generate JARs that contain Smithy models found in Java projects, and generate JARs that contain filtered *projections* of Smithy models.</description>
  <url>https://github.com/awslabs/smithy-gradle-plugin</url>
  <licenses>
    <license>
      <name>Apache License 2.0</name>
      <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
      <distribution>repo</distribution>
    </license>
  </licenses>
  <developers>
    <developer>
      <id>smithy</id>
      <name>Smithy</name>
      <organization>Amazon Web Services</organization>
      <organizationUrl>https://aws.amazon.com</organizationUrl>
      <roles>
        <role>developer</role>
      </roles>
    </developer>
  </developers>
  <scm>
    <url>https://github.com/awslabs/smithy-gradle-plugin.git</url>
  </scm>
  <dependencies>
    <dependency>
      <groupId>software.amazon.smithy</groupId>
      <artifactId>smithy-model</artifactId>
      <version>[1.0, 2.0)</version>
      <scope>runtime</scope>
    </dependency>
    <dependency>
      <groupId>software.amazon.smithy</groupId>
      <artifactId>smithy-build</artifactId>
      <version>[1.0, 2.0)</version>
      <scope>runtime</scope>
    </dependency>
    <dependency>
      <groupId>software.amazon.smithy</groupId>
      <artifactId>smithy-cli</artifactId>
      <version>[1.0, 2.0)</version>
      <scope>runtime</scope>
    </dependency>
  </dependencies>
</project>

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.