Mark ServiceManager as final and change AbstractPluginManager to use composition over inheritance

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RFC

Q	A
Proposed Version(s)	4.0.0
BC Break?	Yes

Goal

Remove inheritance of ServiceManager.

Background

Besides the fact that composition over inheritance should be preferred, the AbstractPluginManager is overriding methods from ServiceManager which should be delegated instead.

Considerations

Upstream projects might type against PluginManagerInterface or the specific plugin manager instance instead of typing against ServiceManager and thus the BC break would be not too big.

Proposal(s)

• create clean ServiceManager instance within __construct and configure that service manager with the config for the plugin manager.
• remove ServiceManager specific public methods
• only support get, has, build as declared within PluginManagerInterface
• create ConfigurableInterface which provides configure method to consume ServiceManagerConfigurationType which passes the argument to the pluginmanager related ServiceManager
• make PluginManagerInterface implement ConfigurableInterface so that all plugin managers have to implement that interface(s) method

Appendix

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ServiceManager is marked as soft-final as of 4.0.0 and will become final class in 5.0.0

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Closed by #179