add json validator on workspaces_agent_config_versions table
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## Description
Based on discussion [here](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163572#note_2098884426), we would like to create jsonb schema validation on the model for both `object` and `object_changed`.
The validation should be simple, and just very high level.
And then we could remove both columns from the ignored_column in test case.
## Acceptance Criteria
- [ ] have the jsonb validation method for both columns
- [ ] remove both columns from the ignored_column
## Technical Requirements
NA
## Design Requirements
NA
## Impact Assessment
NA
## User Story
NA
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