Adds integration with caching plugins purge mechanisms

[abaicus]

All Submissions:

• Have you followed the Optimole Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

• Adds cache purge integration with the following plugins:
  • Super Page Cache & Super Page Cache PRO
  • Cache Enabler
  • Aruba Hispeed Cache
  • Hummingbird

Closes Codeinwp/optimole-service#1595.

How to test the changes in this Pull Request:

1. Install each of the caching plugins;
2. When purging the Optimole image cache from settings the mentioned plugins cache should also be purged

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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