feat: get major Postgres version and tuning params from the source DB (#515)

Description

When testing source connection:

• detect major Postgres version
• get tuning params from the source DB

Add query results to the POST /admin/test-db-source response and update OpenAPI Spec

Related issue

#515 (closed)

Examples

Response of POST /admin/test-db-source

{
  "status": "ok",
  "result": "ok",
  "message": "Database ready for dump and restore",
  "dbVersion": 14,
  "tuningParams": {
    "autovacuum_work_mem": "-1",
    "constraint_exclusion": "partition",
    "cpu_index_tuple_cost": "0.005",
    "cpu_operator_cost": "0.0025",
    "cpu_tuple_cost": "0.01",
    "cursor_tuple_fraction": "0.1",
    "default_statistics_target": "100"
  }
}

Checklist

• MR description has been reviewed
• MR changes are functionally tested
• MR does NOT have API/CLI changes OR there are API/CLI changes and they have been reviewed & DOCS ARE ADJUSTED (reference doc, etc)
• MR does NOT have UI changes OR there are UI changes and they have been reviewed & UX IS REVIEWED

Closes #515 (closed)