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/server/server.go

@@ -19,6 +19,7 @@ import (
 	"github.com/grafana/grafana/pkg/api/routing"
 	"github.com/grafana/grafana/pkg/bus"
 	_ "github.com/grafana/grafana/pkg/extensions"
+	"github.com/grafana/grafana/pkg/infra/httpclient/httpclientprovider"
 	"github.com/grafana/grafana/pkg/infra/localcache"
 	"github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/infra/metrics"
@@ -281,6 +282,7 @@ func (s *Server) buildServiceGraph(services []*registry.Descriptor) error {
 		s.cfg,
 		routing.NewRouteRegister(middleware.ProvideRouteOperationName, middleware.RequestMetrics(s.cfg)),
 		localcache.New(5*time.Minute, 10*time.Minute),
+		httpclientprovider.New(s.cfg),
 		s,
 	}
 	return registry.BuildServiceGraph(objs, services)