Expose Drawing Manager Source for editing shapes in drawing manager from code

[mtech-rherpio]

Problem:
There is currently no way to bring in new shapes to drawing manager or edit drawn shapes from code and reload them back into the drawing manager. This means it's not possible to edit existing shapes without redrawing them from scratch again.

Is the feature already existing on azure-maps-control or on one of its plugins?
Yes, the drawing manager source is accessible in azure-maps-drawing tools module. Example of its use in Azure maps code samples.

Solution:
Add DrawingManager property in Map and Source property in that object. Then it would be ready to be extended with rest of the properties and methods. The property would contain the source used by the drawing manager, and it could be used just like other data sources to update the data inside drawing manager.

Screenshots
drawingManager.getSource in js

[mtech-rherpio]

I started looking into this a bit deeper and making a clean solution like that seems harder than I originally thought, the functionality of the sources is tied to the Map sources list, but the DataSource in the DrawingManager can't be accessed the same way. I managed to add methods for adding shapes to drawing manager and clearing the shapes, which solves my problem, but it's not as good as I hoped.

I can clean up my code and make a PR next week.