Extension Data API - Allow custom widget to add custom data items to Context Data Objects

When building a verse extension "custom widget", we often need to store custom data fields on the email being read or composed.

Currently, these custom data items cannot be written to the email via the "Context Data Object" supplied by verse, so we have to invent other ways of transferring/storing this information, making the widgets more complicated than they need to be.

It would be useful if, for the "mail.read" and "mail.compose" actions, if we could supply a list of custom data items in the "widget configuration" that are populated from the underlying email. This would enable custom data items to be written to an email (as well as reading any existing custom data when displaying the custom widget).