chore: move /gitauth to /externalauth on the frontend (#9954)

* chore: move `/gitauth` to `/externalauth` on the frontend

This actually took a lot more jank than anticipated,
so I wanted to split this up before adding the ability
to embed new providers.

* Rename FE

* Fix em' up

* Fix linting error

* Fix e2e tests

* chore: update helm golden files

coderd/httpmw/externalauthparam.go

@@ -0,0 +1,40 @@
+package httpmw
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"github.com/coder/coder/v2/coderd/externalauth"
+	"github.com/coder/coder/v2/coderd/httpapi"
+)
+
+type externalAuthParamContextKey struct{}
+
+func ExternalAuthParam(r *http.Request) *externalauth.Config {
+	config, ok := r.Context().Value(externalAuthParamContextKey{}).(*externalauth.Config)
+	if !ok {
+		panic("developer error: external auth param middleware not provided")
+	}
+	return config
+}
+
+func ExtractExternalAuthParam(configs []*externalauth.Config) func(next http.Handler) http.Handler {
+	configByID := make(map[string]*externalauth.Config)
+	for _, c := range configs {
+		configByID[c.ID] = c
+	}
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			config, ok := configByID[chi.URLParam(r, "externalauth")]
+			if !ok {
+				httpapi.ResourceNotFound(w)
+				return
+			}
+
+			r = r.WithContext(context.WithValue(r.Context(), externalAuthParamContextKey{}, config))
+			next.ServeHTTP(w, r)
+		})
+	}
+}