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:

It may be useful to start with.

TODO:

• Find where in the spec the Uri type is defined for the packaging spec