fix: Prevent Memory leaks from sentry-tower

[Swatinem]

The TowerService::call seems to just create futures, and is not running in the context of a parent layers future. Also the other way around, SentryService didn’t wrap the call in a Hub either.

[loewenheim]

Let me see if I understand this correctly. You're moving some code from the function that creates the future to the first poll of the future because the way it was originally, the closure in sentry::configure_scope would be kept around too long?

[Swatinem]

Its rather 2 issues:

1. The "caller" in the SentryService did not wrap the actual call function with a hub, wrongly assuming that every async fn is lazy.
2. The "callee" in the SentryHttpService was executing synchronous code in its own call, violating the assumption that async fn should be lazy.