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/services/datasourceproxy/datasourceproxy.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/grafana/grafana/pkg/api/datasource"
 	"github.com/grafana/grafana/pkg/api/pluginproxy"
+	"github.com/grafana/grafana/pkg/infra/httpclient"
 	"github.com/grafana/grafana/pkg/infra/metrics"
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/plugins"
@@ -25,6 +26,7 @@ type DatasourceProxyService struct {
 	PluginRequestValidator models.PluginRequestValidator `inject:""`
 	PluginManager          plugins.Manager               `inject:""`
 	Cfg                    *setting.Cfg                  `inject:""`
+	HTTPClientProvider     httpclient.Provider           `inject:""`
 }
 
 func (p *DatasourceProxyService) Init() error {
@@ -62,7 +64,7 @@ func (p *DatasourceProxyService) ProxyDatasourceRequestWithID(c *models.ReqConte
 	}
 
 	proxyPath := getProxyPath(c)
-	proxy, err := pluginproxy.NewDataSourceProxy(ds, plugin, c, proxyPath, p.Cfg)
+	proxy, err := pluginproxy.NewDataSourceProxy(ds, plugin, c, proxyPath, p.Cfg, p.HTTPClientProvider)
 	if err != nil {
 		if errors.Is(err, datasource.URLValidationError{}) {
 			c.JsonApiErr(http.StatusBadRequest, fmt.Sprintf("Invalid data source URL: %q", ds.Url), err)