refactor(jans-cedarling): consolidate JwtService initialization logic into the module

[rmarinn]

Is your feature request related to a problem? Please describe.

In #9870, the initialization logic for JwtService was moved outside its module, resulting in a less modular and more tangled initialization process. This not only makes JwtService harder to maintain, but also introduces coupling between multiple modules. Whenever the constructor for JwtService changes, those changes cascade across the init and common modules, causing unintended dependencies and increasing the complexity of modifications.

Describe the solution you'd like

• The JwtService initialization logic should reside entirely within its own module. This ensures that other modules are not responsible for managing how JwtService is set up, reducing unnecessary coupling.
• To achieve this, an interface will be implemented for JwtService that other modules can rely on for interaction. This interface will abstract the initialization details, promoting decoupling. This means other modules will be insulated from changes in JwtService's constructor or internal setup, allowing for easier modifications and better separation of concerns.