Support configuring multiple repositories

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:

~~~ xml

myChartRepo
https://example.github.io/chart-repo/

https://example.com/helm
PUT

~~~

Start by reading the PackageMojo step that adds the incubator repository and the configuration example in this issue. Define how a list of named Helm repositories, optional credentials, and a separate publish repository should be represented and consumed. Done means the plugin makes no assumptions about incubator or regular repositories and supports the requested publishing configuration.

Written by the indexing model from the issue text.