Fix review comments

- Combine the various exported types; the only API surface is now the
  `PluginDatabaseClientFactory` which is a free function.
- Rename to the SingleConnectionManager class.
- Refactor the PluginEnvironment type to use databaseClientFactory for
  clarity.
- Fix mergeDatabaseConfig to not override the input dictionary.

plugins/auth-backend/src/service/router.ts

@@ -24,12 +24,12 @@ import { DatabaseKeyStore, TokenFactory, createOidcRouter } from '../identity';
 import {
   NotFoundError,
   PluginEndpointDiscovery,
-  PluginDatabaseFactory,
+  PluginDatabaseClientFactory,
 } from '@backstage/backend-common';
 
 export interface RouterOptions {
   logger: Logger;
-  database: PluginDatabaseFactory;
+  database: PluginDatabaseClientFactory;
   config: Config;
   discovery: PluginEndpointDiscovery;
 }
@@ -48,7 +48,7 @@ export async function createRouter({
   const keyDurationSeconds = 3600;
 
   const keyStore = await DatabaseKeyStore.create({
-    database: await database.getDatabase(),
+    database: await database(),
   });
   const tokenIssuer = new TokenFactory({
     issuer: authUrl,

plugins/auth-backend/src/service/standaloneServer.ts

@@ -53,10 +53,8 @@ export async function startStandaloneServer(
   const router = await createRouter({
     logger,
     config,
-    database: {
-      async getDatabase(_database?: string) {
-        return database;
-      },
+    database: async () => {
+      return database;
     },
     discovery,
   });