feat: implement ListActivities pagination

[biplavbarua]

This PR implements pagination for the ListActivities API using page_size and page_token.

Changes

• Store: Added Limit and Offset to FindActivity struct.
• Drivers: Updated SQLite, MySQL, and PostgreSQL drivers to support pagination queries.
• Service: Updated ListActivities in activity_service.go to handle pagination parameters and generate next_page_token.
• Tests: Added activity_service_test.go to verify pagination logic.

Verification

• Added new unit test TestListActivities covering creation and pagination flows.
• Verified locally with go test.