support nestjs v8 NestInterceptor interface

[jonapgartwohat]

In version 8 of nestjs, the intercept method of the NestInterceptor interface has a new signature:

intercept(context: ExecutionContext, next: CallHandler<T>): Observable<R> | Promise<Observable<R>>;

Prior to this PR, the intercept method in DataLoaderInterceptor was incompatible, yielding the following TS error.

Type 'Observable<any>' is not assignable to type 'Observable<any> | Promise<Observable<any>>'.

I also removed the rxjs dependency as it's no longer needed.

fixes: #50

[jonapgartwohat]

closing in favor of #51