feat: Add possibility to specify extra providers

[Gamote]

Useful when the consumers wants to use a provider in the useFactory method. Inspired by Nest's HttpModule (code).
More info on the issue: https://stackoverflow.com/a/66164772/10878244

Usage example:

import ...

@Global()
@Module({})
export class AppMailerModule {
  public static registerAsync(options: AppMailerAsyncOptions): DynamicModule {
    const configProvider = this.createConfigProvider(options);

    return {
      module: AppMailerModule,
      imports: [
        MailerModule.forRootAsync({
          imports: options.imports || [],
          inject: [APP_MAILER_OPTIONS],
          extraProviders: [configProvider], // <-- Specify extra providers
          useFactory: async (consumerOptions: AppMailerOptions) => {
            return {
              ...consumerOptions.mailer,
              // <-- Now we can use it to add default options for example
            };
          },
        }),
        ...(options.imports || []),
      ],
      providers: [configProvider, AppMailerService],
      exports: [AppMailerService],
    };
  }
}

[Gamote]

@cdiaz @juandav Is there anything I can do to make it easier for you to review this PR?