Investigate ITargetFeature use to redirect target path for EmbeddedPackage and EmbeddedObject (and other) parts.

From #1318.

TLDR:
We should investigate using the features (perhaps ITargetFeature) to customize added parts like `EmbeddedPackagePart` and `ImagePart` where the target path was specified as a compromise between the apps' specific structures so that we can mimic Excel, Word and PowerPoint's exact targeting behaviors.

MORE:

I was looking at this and I see why `EmbeddedPackagePart`s are saved to a new `embeddings` folder in the folder of the referencing part. `EmbeddedPackagePart` is a shared type in ISO 29500-1. That means that it is a target of an explicit relationship of several parts in WordprocessingML, SpreadsheetML and PresentationML documents. However, these documents don't have the same structures wrt where those referencing parts live path-wise. So when the SDK designers were considering how to declaratively specify using XML definitions with a targetPath specification like `embeddings` or `../embeddings` or `/embeddings`, they couldn't come up with one that would satisfy all three apps. For Excel, it would be `../embeddings` but for Word it would be `./embeddings` or just `embeddings`. And PowerPoint is one of those two options. So the alternative was to write front end code or generate more back end code for the front end to mimic the behavior of the three apps. When considering this with all the other behaviors that don't actually impact the consumption of the package by those apps, they opted to just pick one, effectively `./embeddings`, and call it a day. I'm sure there are tons of other behaviors like this in the SDK. "Correcting" them all to mimic Office exactly starts to bloat the SDK and approach a large codebase like the apps themselves.

Effectively this is an enhancement to the SDK, vs a bug fix since the produced packages are not broken now. Regarding using features, I see that we generate:

```csharp
private sealed class GeneratedFeatures : TypedPartFeatureCollection, ITargetFeature
{
public GeneratedFeatures(OpenXmlPart part) : base(part) { }
string ITargetFeature.Extension => ".bin";
string ITargetFeature.Name => "image";
string ITargetFeature.Path => "../media";
}
```

So perhaps we can use ITargetFeature to customize this behavior.