Introduce Symfony events to allow altering the subscription lifecycle [#3592892]

Problem/Motivation

Currently, the module's infrastructure is completely self-contained and does not allow any other module to dynamically modify subscription payload or lifecycle behavior. This limits the module's extensibility and prevents developers from implementing custom business logic required for real-world projects.

Proposed resolution

Introduce an event-driven architecture so other modules can intercept and modify the PayPal subscription process.

Issue fork 3592595-3592892

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3592892-introduce-symfony-events changes, plain diff MR !2
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