Add Postgres connection validation health event

[sethsamuel]

What does this PR do?

• Adds a validation step on check init to verify that the Postgres connection is valid and the correct permissions are applied
• Emits an agent health event after this validation

Motivation

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• [ ] If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov[bot]]

Codecov Report

:x: Patch coverage is 90.62500% with 9 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.30%. Comparing base (a32934f) to head (a0799f1). :warning: Report is 2 commits behind head on master.

Additional details and impacted files

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