Extend `XmlTasks` with a new extension for adding nodes

[ITaluone]

Description

In my day work I came across the issue to add some intermediate nuget feed (usually the local Artifacts directory) for e.g. smoke tests.

Now, I have to do this myself. Would be convenient, if Fallout would have this built in.

Usage Example

XmlTasks.XmlAdd(RootDirectory / "nuget.config", "/configuration/packageSources", new XmlElementBuilder("add")
            .SetAttribute("key", "local")
            .SetAttribute("value", "/tmp/local/inplace/nuget/feed")
            .SetAttribute("allowInsecureConnections", "true"));

Or similar

Alternative

Doing this on my own

Could you help with a pull-request?

Yes

[ChrisonSimtian]

nice I like it, we have exactly the same scenario at work. Every repo got its own nuget.config, I got everything in one folder with its own (because why wait for the CI to finish when you can build locally and run your own nuget server, right?)
Love it!