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/services/serviceaccounts/api/api_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/services/accesscontrol"
 	accesscontrolmock "github.com/grafana/grafana/pkg/services/accesscontrol/mock"
+	"github.com/grafana/grafana/pkg/services/contexthandler/ctxkey"
 	"github.com/grafana/grafana/pkg/services/featuremgmt"
 	"github.com/grafana/grafana/pkg/services/serviceaccounts"
 	"github.com/grafana/grafana/pkg/services/serviceaccounts/database"
@@ -238,6 +239,9 @@ func setupTestServer(t *testing.T, svc *tests.ServiceAccountMock,
 			Logger:       log.New("serviceaccounts-test"),
 		}
 		c.Map(ctx)
+
+		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), ctx))
+		c.Map(c.Req)
 	})
 	a.RouterRegister.Register(m.Router)
 	return m, a