feat: add dry run and individual db config parameters

Add a dry-run mode that validates connections and counts source rows without writing, and accept per-connection host/port/user/password/options alongside URLs.

.env.example

@@ -1,6 +1,23 @@
 SOURCE_DB_URL=sqlserver://sa:password@localhost:1433?database=master&packet+size=32767&loc=UTC&dial+timeout=120&connection+timeout=120&KeepAlive=30
+
+# used only when SOURCE_DB_URL is not set
+# SOURCE_DB_HOST=localhost
+# SOURCE_DB_PORT=1433
+# SOURCE_DB_NAME=master
+# SOURCE_DB_USER=sa
+# SOURCE_DB_PWD=secure_password!123
+# SOURCE_DB_OPTIONS="packet+size=32767&loc=UTC&dial+timeout=120&connection+timeout=120&KeepAlive=30"
+
 TARGET_DB_URL=postgresql://postgres:password@localhost:5432/db
 
+# used only when TARGET_DB_URL is not set
+# TARGET_DB_HOST=localhost
+# TARGET_DB_PORT=5432
+# TARGET_DB_NAME=db
+# TARGET_DB_USER=postgres
+# TARGET_DB_PWD=secure_password!123
+# TARGET_DB_OPTIONS=""
+
 LOG_LEVEL=INFO
 
 AZ_STORAGE_ENABLED=false