Support configuring multiple repositories

[pvorb]

Currently, there's a somewhat random step in the PackageMojo that adds an incubator repository. #77 allows us to disable this step, which was necessary for a CI pipeline that is behind a very restrictive firewall that doesn't allow access to the publicly available incubator repository.

In the long term, it would be way nicer if we would not make any assumptions about incubator or regular repositories. We could just support configuring a list of repositories and their alias, as well as a separate repository for deploying/publishing a chart.

This could look similar to the following lines:

<configuration>
    <helmRepositories>
        <helmRepository>
            <name>myChartRepo</name>
            <url>https://example.github.io/chart-repo/</url>
            <!-- optionally with a way to specify credentials -->
        </helmRepository>
    </helmRepositories>

    <publishRepository>
        <url>https://example.com/helm</url>
        <method>PUT</method>
        <!-- optionally with a way to specify credentials -->
    </publishRepository>
</configuration>