Add DBM Postgres setup module and extract configure_connection

[sangeetashivaji]

What does this PR do?

Hosts the DBM Postgres setup logic in the integration so the
datadog-agent integration setup postgres CLI can tunnel into it
(python -m datadog_checks.postgres.setup) rather than shipping a standalone
copy in the agent.

• Adds datadog_checks/postgres/setup.py — the Detect → Plan → Apply logic that
  idempotently configures a Postgres instance for DBM (user, grants, server
  settings, per-database extension/schema/helper functions). Reads a JSON config
  from stdin and writes a JSON result to stdout.
• Extracts configure_connection out of LRUConnectionPoolManager._configure_connection
  into a module-level function so setup configures connections exactly like the
  running check (autocommit, SQL_ASCII decoding, CommenterCursor) without a pool
  instance. The pool manager now delegates to it; behavior is unchanged.
• Setup reuses the integration's building blocks instead of duplicating them:
  configure_connection, AUTODISCOVERY_QUERY (database navigation for
  --all-databases), and parse_shared_preload_libraries.
• Keeps the canonical datadog.* helper-function SQL as constants alongside
  tests/compose/resources/03_setup.sh, giving a single source of truth in this
  repo (resolves the SQL-drift risk).
• On RDS/Aurora re-runs, skips the parameter-group manual step once the value
  already matches, so an already-configured instance verifies cleanly instead of
  reporting manual steps forever.

Motivation

Review feedback on DataDog/datadog-agent#51751: the setup logic belongs in the
Python integration where the connection-management and database-navigation code
already lives, with the Go agent acting only as the tunnel to it.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• 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

[datadog-prod-us1-4]

 

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 70.35%
• Overall Coverage: 93.39%

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[dd-octo-sts]

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