Add ability to set ElementId identifier to a BHoMObject

#### Description:

This request appeared during a discussion with one of the advanced Revit/GH users, who wanted to update a Revit element with a nonrelated BHoM object - to achieve that, we needed to:
1. Pull Revit element
2. Extract the `RevitIdentifiers` fragment from the pulled BHoM object
3. Copy the above fragment to the object the user wants to push
4. Push/Update the new object

It is hacky, but to make hacking easier, there could be a `SetElementId` method that would create a new `RevitIdentifiers` fragment with only `ElementId` property set, which would allow avoiding steps 1. and 2. from the above and making 3. easier.