pkg/web: restrict handler types (#48495)

Makes `pkg/web` only accept handles from the following set:

```go
	handlerStd       = func(http.ResponseWriter, *http.Request)
	handlerStdCtx    = func(http.ResponseWriter, *http.Request, *web.Context)
	handlerStdReqCtx = func(http.ResponseWriter, *http.Request, *models.ReqContext)
	handlerReqCtx    = func(*models.ReqContext)
	handlerReqCtxRes = func(*models.ReqContext) Response
	handlerCtx       = func(*web.Context)
```

This is a first step to reducing above set to only `http.Handler`.

---

Due to a cyclic import situation between `pkg/models` and `pkg/web`, parts of this PR were put into `pkg/api/response`, even though they definitely do not belong there. This however is _temporary_ until we untangle `models.ReqContext`.

pkg/api/response/web_hack.go

@@ -0,0 +1,62 @@
+//nolint:unused,deadcode
+package response
+
+//NOTE: This file belongs into pkg/web, but due to cyclic imports that are hard to resolve at the current time, it temporarily lives here.
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/grafana/grafana/pkg/models"
+	"github.com/grafana/grafana/pkg/services/contexthandler/ctxkey"
+	"github.com/grafana/grafana/pkg/web"
+)
+
+type (
+	handlerStd       = func(http.ResponseWriter, *http.Request)
+	handlerStdCtx    = func(http.ResponseWriter, *http.Request, *web.Context)
+	handlerStdReqCtx = func(http.ResponseWriter, *http.Request, *models.ReqContext)
+	handlerReqCtx    = func(*models.ReqContext)
+	handlerReqCtxRes = func(*models.ReqContext) Response
+	handlerCtx       = func(*web.Context)
+)
+
+func wrap_handler(h web.Handler) http.HandlerFunc {
+	switch handle := h.(type) {
+	case handlerStd:
+		return handle
+	case handlerStdCtx:
+		return func(w http.ResponseWriter, r *http.Request) {
+			handle(w, r, web.FromContext(r.Context()))
+		}
+	case handlerStdReqCtx:
+		return func(w http.ResponseWriter, r *http.Request) {
+			handle(w, r, getReqCtx(r.Context()))
+		}
+	case handlerReqCtx:
+		return func(w http.ResponseWriter, r *http.Request) {
+			handle(getReqCtx(r.Context()))
+		}
+	case handlerReqCtxRes:
+		return func(w http.ResponseWriter, r *http.Request) {
+			ctx := getReqCtx(r.Context())
+			res := handle(ctx)
+			res.WriteTo(ctx)
+		}
+	case handlerCtx:
+		return func(w http.ResponseWriter, r *http.Request) {
+			handle(web.FromContext(r.Context()))
+		}
+	}
+
+	panic(fmt.Sprintf("unexpected handler type: %T", h))
+}
+
+func getReqCtx(ctx context.Context) *models.ReqContext {
+	reqCtx, ok := ctx.Value(ctxkey.Key{}).(*models.ReqContext)
+	if !ok {
+		panic("no *models.ReqContext found")
+	}
+	return reqCtx
+}