Replace deprecated usage of CloudinaryContext

## Bug report for Cloudinary Flutter SDK
The usage of the class CloudinaryContext has been marked as deprecated and there is a suggestion to use the CloudinaryObject instead. But, inside `CldImageWidget`and `CldVideoController` there is usage of the CloudinaryContext to get the Cloudinary object reference.

I believe this should be replaced.

## Describe the bug in a sentence or two.



## Issue Type (Can be multiple)
[ ] Build - Can’t install or import the SDK
[ ] Performance - Performance issues
[ ] Behaviour - Functions aren’t working as expected (Such as generate URL)
[X] Documentation - Inconsistency between the docs and behaviour
[X] Other (Specify)

## Steps to reproduce
N/A

## Error screenshots or Stack Trace (if applicable)
N/A

## Build/Dependency management
[ ] Cocoa-Pods
[ ] Carthage
[ ] Manual import
[ ] Other (Specify)

## Is the issue reproducible only on a specific device?
[X] No
[ ] Yes (specify device model + iOS/Android version)

## Versions and Libraries (fill in the version numbers)
Unrelated

Relates to #31