Add Deprecation headers for legacy rest endpoints (#7686)

* add deprecation headers for legacy rest endpoints

* add deprecation headers for missing tx routes

* rm handler-level deprecation headers

* switch to middleware Route.Use method for setting deprecation Headers

* set deprecation headers using subrouter

* cleanup gofmt

* goimports

* Update client/rest/rest.go

* update deprecation headers to be set on each module individually

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

client/rest/rest.go

@@ -0,0 +1,27 @@
+package rest
+
+import (
+	"net/http"
+
+	"github.com/gorilla/mux"
+)
+
+// addHTTPDeprecationHeaders is a mux middleware function for adding HTTP
+// Deprecation headers to a http handler
+func addHTTPDeprecationHeaders(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Deprecation", "true")
+		w.Header().Set("Link", "<https://docs.cosmos.network/v0.40/interfaces/rest.html>; rel=\"deprecation\"")
+		w.Header().Set("Warning", "199 - \"this endpoint is deprecated and may not work as before, see deprecation link for more info\"")
+		h.ServeHTTP(w, r)
+	})
+}
+
+// WithHTTPDeprecationHeaders returns a new *mux.Router, identical to its input
+// but with the addition of HTTP Deprecation headers. This is used to mark legacy
+// amino REST endpoints as deprecated in the REST API.
+func WithHTTPDeprecationHeaders(r *mux.Router) *mux.Router {
+	subRouter := r.NewRoute().Subrouter()
+	subRouter.Use(addHTTPDeprecationHeaders)
+	return subRouter
+}