Enable acquiring base64 from an image

[dxshindeo]

I didn't see this anywhere in code. I wish to send an image to server and I am going to use base64 format. Nativescripts' imageSource object has a method toBase64String. Currently, there are still image memory leaks the moment I initialise an imageSource object to get base64 data.
Since fresco cleans after itself I thought I would get base64 from fresco object, but there is no such functionality.

[VladimirAmiorkov]

@dxshindeo Hi in the current 3.0.2 version of the plugin, using the FrescoDrawee to get its source is not available as a build-in feature.

I have not researched if this is possible in the native Fresco Android library but if it is we will be glad to approve a PR for it or some reference links.