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.go

@@ -0,0 +1,30 @@
+package httpclientprovider
+
+import (
+	"fmt"
+
+	sdkhttpclient "github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+	"github.com/grafana/grafana/pkg/infra/httpclient"
+	"github.com/grafana/grafana/pkg/setting"
+)
+
+var newProviderFunc = sdkhttpclient.NewProvider
+
+// New creates a new HTTP client provider with pre-configured middlewares.
+func New(cfg *setting.Cfg) httpclient.Provider {
+	userAgent := fmt.Sprintf("Grafana/%s", cfg.BuildVersion)
+	middlewares := []sdkhttpclient.Middleware{
+		DataSourceMetricsMiddleware(),
+		SetUserAgentMiddleware(userAgent),
+		sdkhttpclient.BasicAuthenticationMiddleware(),
+		sdkhttpclient.CustomHeadersMiddleware(),
+	}
+
+	if cfg.SigV4AuthEnabled {
+		middlewares = append(middlewares, SigV4Middleware())
+	}
+
+	return newProviderFunc(sdkhttpclient.ProviderOptions{
+		Middlewares: middlewares,
+	})
+}