Implement `:aggregate` with COUNT support

[rauann]

Summary

Implement aggregate operations, starting with COUNT support using DynamoDB's Select: COUNT.

Context

DynamoDB supports returning only the count of matching items instead of the items themselves. This would enable Ash aggregate queries for counting records.

Implementation Notes

• Use Select: COUNT in Query/Scan operations
• Returns Count and ScannedCount in the response
• For large datasets, may need to handle pagination to get accurate total count
• Consider which Ash aggregate types can be supported (COUNT is native, others may need client-side computation)

References

• DynamoDB Select Parameter