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/testing.go

@@ -0,0 +1,18 @@
+package httpclientprovider
+
+import (
+	"net/http"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+)
+
+type testContext struct {
+	callChain []string
+}
+
+func (c *testContext) createRoundTripper(name string) http.RoundTripper {
+	return httpclient.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+		c.callChain = append(c.callChain, name)
+		return &http.Response{StatusCode: http.StatusOK}, nil
+	})
+}