impl: add databasePicker component logic

[VH3956]

Description: This PR implements the React state and rendering logic for the custom DatabasePicker field extension in the Backstage Scaffolder. This component streamlines the UI by only capturing the dbType and dbName. Secret generation and other connection details are handled securely by the backend operator.

Changes Made:

1. Created Custom Field Extension (packages/app/src/scaffolder/DatabasePickerExtension):

   • DatabasePicker.tsx: Implemented a React component with a dropdown for Database Type ("No Database", "PostgreSQL"). Conditionally renders a text input for Database Name when "PostgreSQL" is selected.

   • extension.ts: Registered the component as a Backstage Scaffolder Field Extension and added custom validation logic (ensures dbName is provided if postgres is selected).

   • index.ts: Exported the new extension.

2. App Integration (packages/app/src/App.tsx):

   • Imported and registered DatabasePickerExtension within the route so Backstage can recognize the custom field.

3. Template Update (examples/advanced-template/template.yaml):

   • Replaced the pure JSON Schema conditional block with the new custom field: ui:field: DatabasePicker. The form now directly passes the { dbType, dbName } object back to the template state.

[hoangphuc841]

Unused scaffolderFieldExtensions Array in App.tsx (Line 79)
const scaffolderFieldExtensions = [DatabasePickerExtension]; // ← Declared but never used
The extension is correctly registered via JSX on line 103 so this array is dead code. Remove it.