Added a new 'Expense Tracker' challenge

[marcelloraffaele]

This pull request introduces a new coding challenge, "Expense Tracker," to the repository. The challenge involves building a microservice for logging expenses and visualizing spending patterns. Key changes include adding the challenge to the README.md file and creating a detailed challenge description in its own README.md.

Addition of the "Expense Tracker" Challenge:

• README.md: Added a link to the "Expense Tracker" challenge under the list of available challenges.
• challenges/expensetracker/README.md: Created a comprehensive guide for the "Expense Tracker" challenge, including problem description, core requirements, user stories, implementation hints, estimated time breakdown, and extra features for participants who finish early.

[marcelloraffaele]

@microsoft-github-policy-service agree

[marcelloraffaele]

I am a CSA, I extended the repository with a new challenge. I found it useful i hope also my colleagues will find this useful too.

[cmendible]

LGTM