Datasource: Improve default timeout settings for HTTP client provider (#36621)

Make sure that default timeout settings are based on configuration
parameters. This now applies for core data sources using old TSDB
contracts and new SDK contracts. Before it was only applied for old TSDB
contracts.
Also moves global setting variables to non-global (setting.Cfg).

pkg/models/datasource_cache_test.go

@@ -312,14 +312,19 @@ func TestDataSource_GetHttpTransport(t *testing.T) {
 }
 
 func TestDataSource_getTimeout(t *testing.T) {
-	setting.DataProxyTimeout = 30
+	originalTimeout := sdkhttpclient.DefaultTimeoutOptions.Timeout
+	sdkhttpclient.DefaultTimeoutOptions.Timeout = 60 * time.Second
+	t.Cleanup(func() {
+		sdkhttpclient.DefaultTimeoutOptions.Timeout = originalTimeout
+	})
+
 	testCases := []struct {
 		jsonData        *simplejson.Json
 		expectedTimeout time.Duration
 	}{
-		{jsonData: simplejson.New(), expectedTimeout: 30 * time.Second},
-		{jsonData: simplejson.NewFromAny(map[string]interface{}{"timeout": nil}), expectedTimeout: 30 * time.Second},
-		{jsonData: simplejson.NewFromAny(map[string]interface{}{"timeout": 0}), expectedTimeout: 30 * time.Second},
+		{jsonData: simplejson.New(), expectedTimeout: 60 * time.Second},
+		{jsonData: simplejson.NewFromAny(map[string]interface{}{"timeout": nil}), expectedTimeout: 60 * time.Second},
+		{jsonData: simplejson.NewFromAny(map[string]interface{}{"timeout": 0}), expectedTimeout: 60 * time.Second},
 		{jsonData: simplejson.NewFromAny(map[string]interface{}{"timeout": 1}), expectedTimeout: time.Second},
 		{jsonData: simplejson.NewFromAny(map[string]interface{}{"timeout": "2"}), expectedTimeout: 2 * time.Second},
 	}