feat(data-events): unique event key

[miguelpeixe]

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Changes proposed in this Pull Request:

Implement support for a unique event key to prevent multiple consecutive dispatches for events that always send the latest and complete payload in the same execution.

The key is determined when registering the action or listener:

Data_Events::register_action( 'my_action', 'email' );
// or:
Data_Events::register_listener( 'my_hook', 'my_action', 'my_callback', 'email' );

In the example above, events dispatched matching the value in the property email of its payload data will only be executed once with the final payload data:

Data_Events::dispatch( $action_name, [ 'email' => 'test@test.com', 'foo' => 'bar' ] );
Data_Events::dispatch( $action_name, [ 'email' => 'test@test.com', 'foo' => 'baz' ] );

Only 1 dispatch will be executed, containing [ 'email' => 'test@test.com', 'foo' => 'baz' ].

This PR depends on #3616

How to test the changes in this Pull Request:

Other information:

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• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[miguelpeixe]

After some thought, this feature seems overkill and adds unnecessary complexity to data events. Each feature can handle their own deduplication to avoid consecutive requests or dispatches.

This is currently being done in Network's content distribution and soon we'll have something similar for ESP syncs.