feat: Add tower integration

[Tuetuopay]

This MR adds a new sentry-tower subcrate which provides tower layers intended to bind Sentry hub to each request. This avoids mixing up breadcrumbs in each sentry event, to only keep those in the logical path that led to the captured event. It is best used in conjunction with sentry-log or sentry-tracing to automatically collect logs to breadcrumbs.

It provides a ready to use NewSentryLayer, that creates a new hub from the currently active one and binds it for the duration of the service handler. It is based upon the SentryLayer which is much more flexible, as it can take a hub directly or a closure producing hub. See the lib.rs and test for usage example.

Note that this could be expanded to automatically capture errors returned by the service, should it fail, though more design thought should be put into it.

[Tuetuopay]

Note about the clippy lint: it is bogus, the suggested fix does not compiles:

error: implementation of `std::ops::FnOnce` is not general enough
   --> sentry-tower/src/lib.rs:143:9
    |
143 |         Hub::with(Hub::new_from_top).into()
    |         ^^^^^^^^^ implementation of `std::ops::FnOnce` is not general enough
    |
    = note: `fn(&'2 std::sync::Arc<sentry_core::Hub>) -> sentry_core::Hub {sentry_core::Hub::new_from_top::<&'2 std::sync::Arc<sentry_core::Hub>>}` must implement `std::ops::FnOnce<(&'1 std::sync::Arc<sentry_core::Hub>,)>`, for any lifetime `'1`...
    = note: ...but it actually implements `std::ops::FnOnce<(&'2 std::sync::Arc<sentry_core::Hub>,)>`, for some specific lifetime `'2`

[Swatinem]

This looks very good, thank you! Just some minor comments.