Replace usage of http.DefaultClient and http.DefaultTransport (#104135)

Remove usage of http.DefaultClient and http.DefaultTransport

Part of grafana/data-sources#484

pkg/util/httpclient/client.go

@@ -0,0 +1,36 @@
+package httpclient
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"time"
+)
+
+// New creates a new http.Client.
+func New() *http.Client {
+	return &http.Client{
+		Transport: NewHTTPTransport(),
+	}
+}
+
+// NewHTTPTransport returns a new HTTP Transport, based off the definition in
+// the stdlib http.DefaultTransport. It's not a clone, because that would return
+// any mutations of http.DefaultTransport from other code at the time of the call.
+// Any plugin that needs a default http transport should use this function.
+func NewHTTPTransport() http.RoundTripper {
+	return &http.Transport{
+		Proxy: http.ProxyFromEnvironment,
+		DialContext: func(dialer *net.Dialer) func(context.Context, string, string) (net.Conn, error) {
+			return dialer.DialContext
+		}(&net.Dialer{
+			Timeout:   30 * time.Second,
+			KeepAlive: 30 * time.Second,
+		}),
+		ForceAttemptHTTP2:     true,
+		MaxIdleConns:          100,
+		IdleConnTimeout:       90 * time.Second,
+		TLSHandshakeTimeout:   10 * time.Second,
+		ExpectContinueTimeout: 1 * time.Second,
+	}
+}