Make Subscription Details Immutable

closes #114

This pull request updates the subscription management system to make subscription details immutable. The key changes are as follows:

TL;DR

• Makes subscription details immutable to ensure consistency and integrity, specially to generate its ID.
• Refactors the subscription structure to separate mutable and immutable fields.
• Updates tests to verify the immutability of subscription details.

Description

New Features and Enhancements

1. Immutability of Subscription Details:

   • Refactored the Subscription struct to separate immutable details from mutable fields.
   • Moved created_at, updated_at, credits, and frequency fields to the main Subscription struct.
   • Ensured that SubscriptionDetails only contains immutable fields like subscriber, target, and metadata.

2. Code Refactoring:

   • Updated various methods to handle the new structure of the Subscription and SubscriptionDetails structs.
   • Improved the update_subscription method to only modify mutable fields and maintain the integrity of immutable details.

3. Testing Enhancements:

   • Added tests to verify that the created_at and subscriber fields remain unchanged during subscription updates.