proxy: v2.127.0 (#5487)

This release adds a new `/shutdown` admin endpoint that may only be
accessed over the loopback network. This allows batch jobs to gracefully
terminate the proxy on completion. The `linkerd-await` utility can be
used to automate this.

This release also includes ~33 non-user-facing changes, the most
important of which updates the Linkerd Transport Header preface to be
`transport.l5d.io/v1\r\n\r\n`.

---

* Simplify orig_proto::Downgrade intializaiton (linkerd/linkerd2-proxy#796)
* Remove unused make_response module (linkerd/linkerd2-proxy#792)
* Add MakeNormalizeUri::layer helper (linkerd/linkerd2-proxy#794)
* Simplify tcp::Forward stack initialization (linkerd/linkerd2-proxy#797)
* Add a NewServeHttp::layer helper (linkerd/linkerd2-proxy#793)
* Simplify outbound endpoint stakc creation (linkerd/linkerd2-proxy#798)
* Simplify NewRouter initialization (linkerd/linkerd2-proxy#795)
* Remove unused http::add_header module (linkerd/linkerd2-proxy#800)
* Simplify concurrency-limit initialization (linkerd/linkerd2-proxy#799)
* Remove the strip_header::Layer type (linkerd/linkerd2-proxy#801)
* Simplify http-classify stack initialization (linkerd/linkerd2-proxy#804)
* Simplify HTTP metrics initialization (linkerd/linkerd2-proxy#806)
* Add checks to ensure support for opaque transport (linkerd/linkerd2-proxy#808)
* Remove the override_authority::Layer type (linkerd/linkerd2-proxy#802)
* Simplify tls::client module (linkerd/linkerd2-proxy#809)
* Simplify header_from_target stack initialization (linkerd/linkerd2-proxy#803)
* Cleanup HTTP boxing modules (linkerd/linkerd2-proxy#805)
* Include scope names in failfast error messages (linkerd/linkerd2-proxy#791)
* Simplify drain module implementation (linkerd/linkerd2-proxy#810)
* Simplify BoxNewService initialization (linkerd/linkerd2-proxy#807)
* admin: Add a /shutdown endpoint (linkerd/linkerd2-proxy#811)
* stack: Remove the MakeReady helper (linkerd/linkerd2-proxy#813)
* Remove the stack::Oneshot helper (linkerd/linkerd2-proxy#812)
* http: Simplify http::boxed type references (linkerd/linkerd2-proxy#814)
* Simplify stack type constraints (linkerd/linkerd2-proxy#815)
* Rename transport types to ConnectTcp and BindTcp (linkerd/linkerd2-proxy#819)
* Remove box from inbound connector (linkerd/linkerd2-proxy#816)
* tap: Simplify tap initialization (linkerd/linkerd2-proxy#820)
* Update Rust to 1.49.0 (linkerd/linkerd2-proxy#822)
* Avoid boxing I/O types for optional TLS (linkerd/linkerd2-proxy#817)
* Decouple TLS detection from TCP connections (linkerd/linkerd2-proxy#818)
* stack: Rename NewOptional to NewUnwrapOr (linkerd/linkerd2-proxy#824)
* inbound: Decouple inbound stack from TCP connections (linkerd/linkerd2-proxy#823)
* Rename opaque-transport to transport-header (linkerd/linkerd2-proxy#825)