Support PostMessage properties in CheckFileInfo

[maboivin]

Motivation

• Why is this feature required?
• Is your feature request related to a problem? What problem?

We are trying to use add-ins through the WOPI protocol. To do so, we need to complete the security handshake and the security handshake is done through the PostMessage API as documented here.

To use the PostMessage API, we need to set the PostMessageOrigin property in the CheckFileInfo response as documented here.

All the PostMessage properties are documented here

An extension point should be also created because the check file info object is created by calling the IWopiStorageProvider.GetFile which returns an IWopiFile instance.

A clear and concise description of what the problem is.

The problem is that the CheckFileInfo.cs doesn't have any of the PostMessage properties and there doesn't seem to be an easy way to customize the CheckFileInfo behavior.

Describe the solution you'd like
The CheckFileInfo should implement all the PostMessage properties in the documentation. There should be a way to customize the CheckFileInfo through the IWopiFile or through another method called on the IWopiStorageProvider?

Describe alternatives you've considered

• Create a PR to fix the issue but we're not that familiar with the WOPI protocol yet.
• Fork this repo.
• Rewrite this library in our private repo to use patterns that better suits our needs even though this package has saved us a lot of time in making it work.

Additional context
None.