Add configuration agent health event for Postgres

[sethsamuel]

What does this PR do?

• Migrates Postgres config to the InstanceConfg model
• Adds and extends config validation
• Emits an agent health event when a config is loaded

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]

Codecov Report

❌ Patch coverage is 86.70757% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.23%. Comparing base (df8de38) to head (4cfbfde).

Additional details and impacted files

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

⚠️ The qa/skip-qa label has been added with shippable changes

The following files, which will be shipped with the agent, were modified in this PR and
the qa/skip-qa label has been added.

You can ignore this if you are sure the changes in this PR do not require QA. Otherwise, consider removing the label.

List of modified files that will be shipped with the agent

postgres/changelog.d/21211.added
postgres/datadog_checks/postgres/config.py
postgres/datadog_checks/postgres/config_models/defaults.py
postgres/datadog_checks/postgres/config_models/instance.py
postgres/datadog_checks/postgres/config_models/validators.py
postgres/datadog_checks/postgres/connection_pool.py
postgres/datadog_checks/postgres/data/conf.yaml.example
postgres/datadog_checks/postgres/discovery.py
postgres/datadog_checks/postgres/health.py
postgres/datadog_checks/postgres/metadata.py
postgres/datadog_checks/postgres/metrics_cache.py
postgres/datadog_checks/postgres/postgres.py
postgres/datadog_checks/postgres/relationsmanager.py
postgres/datadog_checks/postgres/statement_samples.py
postgres/datadog_checks/postgres/statements.py