feat: move stakerapp creation from main to stakerservice

[wnjoon]

This PR addresses the // TODO: consider moving this to stakerservice comment in main.go.

Changes:

• Added the NewStakerServiceFromConfig function.
• Within NewStakerServiceFromConfig, the StakerService is now created after calling NewStakerAppFromConfig.

Benefits of this refactoring:

• Separation of Concerns: main.go now primarily serves as a high-level coordinator, reducing its responsibilities and simplifying the code.
• Encapsulation: Future changes to how stakerApp instance is created will no longer require modifications to main.go.
• Improved Testability: By creating the stakerApp instance within stakerService, mock objects can be injected more easily for testing.
• Better Dependency Management: Having stakerService directly instantiate stakerApp clarifies the dependency relationship between the components.