Refactor service providers instantiation & subscribers declaration in the main plugin class

[remyperona]

In the main plugin class:

• We currently instantiate & add to the container each service provider one by one
• We use array of subscribers aliases to add them to the event manager

While it works, it represents a lot of line of codes for something that could be done by using a couple of loops and config files.

This is something we already do in Imagify & RocketCDN, so we can replicate a similar strategy here.

Scope a solution ✅

For the service providers instantiation:

• Add a config directory with a providers.php file which contains an array of ServiceProviders fully-qualified names
• require the file in the main.php file
• pass the value to the load() method
• loop over the providers to create the new instances and pass them to the container
• see https://github.com/wp-media/imagify-plugin/blob/b07b6a9605c096d5963d652b2a630d359c5202ec/classes/Plugin.php#L140

For the subscribers part:

• Create a new interface SubscriberServiceProvider, with a $subscribers array property and a get_subscribers() method that returns the $subscribers
• Update Service Providers that have subscribers (not all of them do), to implement the new interface and populate the $subscribers value
• Update Plugin class to remove the arrays containing the subscriber aliases
• Loop over the subscribers to instantiate them and pass them to the event manager
• See https://github.com/wp-media/imagify-plugin/blob/b07b6a9605c096d5963d652b2a630d359c5202ec/classes/Plugin.php#L246

Estimate the effort ✅

Effort [S]