Add UpdateItem / partial attribute updates to Database service

[NitinKumar004]

Summary

The Database driver interface is missing UpdateItem (partial attribute update) — one of the most frequently used operations in DynamoDB/CosmosDB/Firestore.

Currently, the only way to modify an item is to PutItem with the full document, which overwrites everything. Real applications almost always use partial updates.

What's Missing

• AWS DynamoDB: UpdateItem with UpdateExpression (SET, REMOVE, ADD, DELETE)
• Azure CosmosDB: Partial document update / patch operations
• GCP Firestore: Update with field masks / merge writes

Why This Matters

• UpdateItem is the Add 6 realistic cloud behaviors across AWS, Azure, and GCP providers #1 most-used DynamoDB API after GetItem
• Without it, you can't test atomic counters, conditional updates, or field-level modifications
• Forces test code to read-then-write (PutItem), which doesn't match real app behavior

Acceptance Criteria

• Add UpdateItem to database/driver/driver.go interface
• Implement in providers/aws/dynamodb/
• Implement in providers/azure/cosmosdb/
• Implement in providers/gcp/firestore/
• Support SET (add/overwrite attributes) and REMOVE (delete attributes) expressions
• Add tests covering all 3 providers