features: Make tower-http enable the tower feature (#532)

Previous the `tower-http` feature enabled the `sentry-tower` dependency and its `http` feature, but it did not enable the `tower` feature of the `sentry` crate. This leads to `sentry::integrations::tower` not being available, even though the dependency is enabled correctly.

Author: Turbo87

sentry/Cargo.toml

@@ -33,7 +33,7 @@ debug-images = ["sentry-debug-images"]
 log = ["sentry-log"]
 slog = ["sentry-slog"]
 tower = ["sentry-tower"]
-tower-http = ["sentry-tower", "sentry-tower/http"]
+tower-http = ["tower", "sentry-tower/http"]
 tracing = ["sentry-tracing"]
 profiling = ["sentry-core/profiling"]
 frame-pointer = ["sentry-core/frame-pointer"]