Adds a decorator @SentryTraced() to the nestjs sdk that can be used on functions to wrap them in a span.

Manual test from my sample application see screenshot below.

![Screenshot 2024-07-02 at 13 58 51](https://github.com/getsentry/sentry-javascript/assets/29484629/c739077d-1b07-4d46-bdf1-16b8e7fcc9dc)

+ e2e test
+ README update with short example demonstrating how to use this feature

