[FEATURE] Added image field

This PR:

- Adds an ImageURL field to the IndexMetadata and Document.
- Adds a TestAdapter
- Addsd get/set/has methods to the Document class

BC: NO
Its deliberately made up of single words (no getFoo or inFoo). Its designed to be brief but understandable. If people want I can rename the terminator to "execute". But I am happy with it as it is.