Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

options/locale/locale_en-US.json

@@ -3315,7 +3315,6 @@
   "admin.config.cache_config": "Cache Configuration",
   "admin.config.cache_adapter": "Cache Adapter",
   "admin.config.cache_interval": "Cache Interval",
-  "admin.config.cache_conn": "Cache Connection",
   "admin.config.cache_item_ttl": "Cache Item TTL",
   "admin.config.cache_test": "Test Cache",
   "admin.config.cache_test_failed": "Failed to probe the cache: %v.",
@@ -3330,7 +3329,6 @@
   "admin.config.instance_web_banner.message_placeholder": "Banner message (supports markdown)",
   "admin.config.session_config": "Session Configuration",
   "admin.config.session_provider": "Session Provider",
-  "admin.config.provider_config": "Provider Config",
   "admin.config.cookie_name": "Cookie Name",
   "admin.config.gc_interval_time": "GC Interval Time",
   "admin.config.session_life_time": "Session Life Time",