Add Aleph Credits system with balance tracking and API endpoints

[amalcaraz]

Summary

• Implement Aleph Credits system to track user credit balances from token distributions
• Add new database table credit_balances with migration to store credit transactions
• Create API endpoints for querying credit balances at account and system levels
• Integrate credit balance updates through POST messages from authorized addresses

Changes

• Database: New AlephCreditBalanceDb model and migration for credit balance tracking
• API Endpoints: Added /accounts/{address}/credits and /credits/balances endpoints
• Message Processing: Enhanced POST message handler to process credit distribution messages
• Balance Management: Credit balance accessors with expiration date handling and bulk updates
• Configuration: Added credit balance configuration with authorized addresses

Test plan

• Verify credit balance database operations (insert, query, expiration handling)
• Test API endpoints for individual and paginated credit balance queries
• Validate POST message processing for credit distributions
• Confirm proper handling of expired credits in balance calculations
• Test bulk credit balance updates and conflict resolution

[nesitor]

I think this PR is not base on main, because have code changes from the pricing recalculation, please rebase it on main or change the base branch here on GitHub.
Also, I have put some comments on the credit implementation.