Backend Migration: New field for workspaces_agent_config_version on workspace table

MR: Add column workspaces_agent_config_version and ... (!165912 - merged)

Description

We need a new integer column workspaces_agent_config_version on the workspace table to record which version of agent_config it belongs to.

and the second post_migration for backfill all existing workspaces.

Acceptance Criteria

• have migration MR to introduce the new column
• have the post migration script to update on all values