Use specialized URI type for package spec that is more relaxed than System.Uri

**Is your feature request related to a problem? Please describe.**
The current Uri usage in the System.IO.Packaging is very strict to the official RFC, but appears to be too strict for what Office documents can have

**Describe the solution you'd like**
We should have a generalized type (i.e. `OpenXmlUri`) that will easily convert to System.Uri or string representations.

**Describe alternatives you've considered**
There are a few attempts at handling this, most recently, this:

https://github.com/dotnet/Open-XML-SDK/blob/307fa23e7f92222230216bb73a756022b8104046/src/DocumentFormat.OpenXml.Framework/Packaging/PackageUriHandlingExtensions.cs#L228-L241

It may be useful to start with.

TODO:

- [x] Find where in the spec the Uri type is defined for the packaging spec