Implement 1-Day Caching for getPurchases API Calls

[TanmayDhobale]

Changes Made

• Added an in-memory caching object to store the results of getPurchases calls.
• Implemented logic within getPurchases to check for valid cached data before proceeding with API calls.
• Cached data is stored with a timestamp to ensure it's only used for up to 24 hours, after which it's refreshed on the next call.
• This caching mechanism is applied conditionally, so it only caches data when the environment does not rely on the local CMS provider for course information.

Benefits

• Reduced API Calls: Significantly decreases the number of external API requests, lowering our server load and potential costs.
• Improved Performance: Users will experience faster response times when accessing their purchases, enhancing overall user satisfaction.
• Scalability: Reduces the risk of hitting API rate limits as our user base grows.

Considerations-
The current implementation is an in-memory cache, which means cached data is reset if the server restarts. For long-term scalability, we may consider using a more persistent caching solution like Redis.
-We'll need to monitor the memory usage on our server to ensure the cache does not grow too large.

[hkirat]

Please use https://github.com/code100x/cms/blob/main/src/db/Cache.ts
generify this to take the expiry as an input

[TanmayDhobale]

@hkirat can u pls check #199

[hkirat]

Closing in favor of #208