Separate "resolve" repo from "target" repo config

### New feature, improvement proposal

Background: The plugin needs to update the `maven-metadata.xml` file. Hence it needs to download it from somewhere. This is what I call the "resolve" repo. It is usually the same as the "target" repo. But not in every case.

Suppose the requirement is to use the plugin solely to create local copy of what will (at a later stage) be uploaded to the target repo. Hence, we have:

```xml

local::file:./target/staging-deploy

```

The above setup will (AFAIK) attempt the download the `maven-metadata.xml` file from the above file URL. No such file will be present so it will assume that this is the first version of the GAV to be uploaded and it will therefore create the metadata file accordingly. Which is wrong.

I believe that what we need is a config knob that allows to use another URL for "resolve" of the metadata vs the URL where the upload happens.