Datasource: Shared HTTP client provider for core backend data sources and any data source using the data source proxy (#33439)

Uses new httpclient package from grafana-plugin-sdk-go introduced 
via grafana/grafana-plugin-sdk-go#328. 
Replaces the GetHTTPClient, GetTransport, GetTLSConfig methods defined 
on DataSource model.
Longer-term the goal is to migrate core HTTP backend data sources to use the 
SDK contracts and using httpclient.Provider for creating HTTP clients and such.

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/infra/httpclient/httpclientprovider/http_client_provider_test.go

@@ -0,0 +1,52 @@
+package httpclientprovider
+
+import (
+	"testing"
+
+	sdkhttpclient "github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHTTPClientProvider(t *testing.T) {
+	t.Run("When creating new provider and SigV4 is disabled should apply expected middleware", func(t *testing.T) {
+		origNewProviderFunc := newProviderFunc
+		providerOpts := []sdkhttpclient.ProviderOptions{}
+		newProviderFunc = func(opts ...sdkhttpclient.ProviderOptions) *sdkhttpclient.Provider {
+			providerOpts = opts
+			return nil
+		}
+		t.Cleanup(func() {
+			newProviderFunc = origNewProviderFunc
+		})
+		_ = New(&setting.Cfg{SigV4AuthEnabled: false})
+		require.Len(t, providerOpts, 1)
+		o := providerOpts[0]
+		require.Len(t, o.Middlewares, 4)
+		require.Equal(t, DataSourceMetricsMiddlewareName, o.Middlewares[0].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, SetUserAgentMiddlewareName, o.Middlewares[1].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, sdkhttpclient.BasicAuthenticationMiddlewareName, o.Middlewares[2].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, sdkhttpclient.CustomHeadersMiddlewareName, o.Middlewares[3].(sdkhttpclient.MiddlewareName).MiddlewareName())
+	})
+
+	t.Run("When creating new provider and SigV4 is enabled should apply expected middleware", func(t *testing.T) {
+		origNewProviderFunc := newProviderFunc
+		providerOpts := []sdkhttpclient.ProviderOptions{}
+		newProviderFunc = func(opts ...sdkhttpclient.ProviderOptions) *sdkhttpclient.Provider {
+			providerOpts = opts
+			return nil
+		}
+		t.Cleanup(func() {
+			newProviderFunc = origNewProviderFunc
+		})
+		_ = New(&setting.Cfg{SigV4AuthEnabled: true})
+		require.Len(t, providerOpts, 1)
+		o := providerOpts[0]
+		require.Len(t, o.Middlewares, 5)
+		require.Equal(t, DataSourceMetricsMiddlewareName, o.Middlewares[0].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, SetUserAgentMiddlewareName, o.Middlewares[1].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, sdkhttpclient.BasicAuthenticationMiddlewareName, o.Middlewares[2].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, sdkhttpclient.CustomHeadersMiddlewareName, o.Middlewares[3].(sdkhttpclient.MiddlewareName).MiddlewareName())
+		require.Equal(t, SigV4MiddlewareName, o.Middlewares[4].(sdkhttpclient.MiddlewareName).MiddlewareName())
+	})
+}