add support for extension versions

[jabrena]

Hi,

I am very happy with this plugin but I don´t see a way to see get the same results for maven extensions.

Currently i use the plugin for:

./mvnw versions:display-dependency-updates
./mvnw versions:display-plugin-updates
./mvnw versions:display-property-updates

but how to get information about extensions?

Example:

    <build>
        <extensions>
            <extension>
                <groupId>io.takari.maven</groupId>
                <artifactId>maven-timeline</artifactId>
                <version>2.0.3</version>
            </extension>
            <extension>
                <groupId>org.apache.maven.extensions</groupId>
                <artifactId>maven-build-cache-extension</artifactId>
                <version>1.2.0</version>
            </extension>
        </extensions>
        ...
    </build>

I could be nice to see something like:

./mvnw versions:display-extensions-updates

Many thanks in advance

Juan Antonio