Expose ClsPluginManager or enable adding plugins for features

[JonnyBGod]

It would be great to be able to do something like:

@Module({})
export class MongodbModule {
  static forRoot(
    uri: string,
    dbName: string,
    options?: MongoClientOptions,
    connectionName?: string,
    extra = false,
  ): DynamicModule {
    return {
      module: MongodbModule,
      imports: [
        MongoCoreModule.forRoot(uri, dbName, options, connectionName),
        ClsModule.forFeature({
          plugins: [
            new ClsPluginTransactional({
              adapter: new TransactionalAdapterMongoDB({
                mongoClientToken: getClientToken(connectionName),
              }),
            }),
          ],
        }),
      ],
    }
  }
}

Or

@Module({})
export class MongodbModule {
  static forRoot(
    uri: string,
    dbName: string,
    options?: MongoClientOptions,
    connectionName?: string,
    extra = false,
  ): DynamicModule {
    return {
      module: MongodbModule,
      imports: [
        MongoCoreModule.forRoot(uri, dbName, options, connectionName),
        ClsModule.registerPlugin(
          new ClsPluginTransactional({
            adapter: new TransactionalAdapterMongoDB({
              mongoClientToken: getClientToken(connectionName),
            }),
          }),
        ),
      ],
    }
  }
}

these would be helpfull for library developement.

Note: not sure what would be the implications regarding the "onClsInit" plugin method.

[Papooch]

Second option implemented in #192