feat(@nestjs/config): better generic support for registerAs

Add support to specify the return type of a config factory.

BREAKING CHANGE:

The commit will break the code which already utilizing the generic of
`registerAs` function.

lib/interfaces/config-factory.interface.ts

@@ -1,7 +1,7 @@
-type ConfigFactoryReturnValue =
-  | Record<string, any>
-  | Promise<Record<string, any>>;
+import { ConfigObject } from '../types';
+
+type ConfigFactoryReturnValue<T extends ConfigObject> = T | Promise<T>;
 
 export type ConfigFactory<
-  T extends ConfigFactoryReturnValue = ConfigFactoryReturnValue
-> = () => T;
+  T extends ConfigObject = ConfigObject
+> = () => ConfigFactoryReturnValue<T>;

lib/types/config-object.type.ts

@@ -0,0 +1 @@
+export type ConfigObject = Record<string, any>;

lib/types/index.ts

@@ -1 +1,2 @@
 export * from './config.type';
+export * from './config-object.type';

lib/utils/register-as.util.ts

@@ -3,17 +3,18 @@ import {
   PARTIAL_CONFIGURATION_PROPNAME,
 } from '../config.constants';
 import { ConfigFactory } from '../interfaces';
+import { ConfigObject } from '../types';
 import { getConfigToken } from './get-config-token.util';
 
 export type ConfigFactoryKeyHost = { KEY: string };
 
 /**
  * Registers the configuration object behind a specified token.
  */
-export function registerAs<T extends ConfigFactory = ConfigFactory>(
-  token: string,
-  configFactory: T,
-): T & ConfigFactoryKeyHost {
+export function registerAs<
+  TConfig extends ConfigObject,
+  TFactory extends ConfigFactory = ConfigFactory<TConfig>
+>(token: string, configFactory: TFactory): TFactory & ConfigFactoryKeyHost {
   Object.defineProperty(configFactory, PARTIAL_CONFIGURATION_KEY, {
     configurable: false,
     enumerable: false,
@@ -27,5 +28,5 @@ export function registerAs<T extends ConfigFactory = ConfigFactory>(
     writable: false,
   });
 
-  return configFactory as T & ConfigFactoryKeyHost;
+  return configFactory as TFactory & ConfigFactoryKeyHost;
 }