sentry: Add tower-axum-matched-path feature

[Turbo87]

#570 introduced the corresponding feature on the sentry-tower crate, but it isn't really useable directly from the sentry crates, which I assume is the recommended way of using these integration crates?

This PR introduces a tower-axum-matched-path feature on the top-level sentry crate to fix this. I'm not a huge fan of the long feature name, but I couldn't think of anything more suitable 😅

/cc @Swatinem

[Swatinem]

well yes, in hindsight, splitting out every integration from the main crate might not have been the best decision :-D