sentry-tower: Use axum::MatchedPath for transaction names

[Turbo87]

If the axum-matched-path feature of sentry-tower is used, it will enable the corresponding matched-path feature of axum and use the MatchedPath request extension to generate the transaction name. As before, the HTTP method is also still included in the transaction name.

Resolves #541

/cc @Swatinem

[Turbo87]

for context: this is supposed to replace https://github.com/rust-lang/crates.io/blob/8b72b4ad67bab12574f7eb8704f8997c047054f1/src/middleware/sentry.rs, but it has the advantage that the trace_sampler will get the MatchedPath value too, instead of the full URL, which makes it easier to make sampling decisions based on the URL.

[Swatinem]

For context, we also have this workaround here: https://github.com/getsentry/symbolicator/blob/8c3ee0db76649706b8f3363a030d84b6a9c7763f/crates/symbolicator/src/endpoints/requests.rs#L21