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/user_agent_middleware.go

@@ -0,0 +1,27 @@
+package httpclientprovider
+
+import (
+	"net/http"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+)
+
+// SetUserAgentMiddlewareName is the middleware name used by SetUserAgentMiddleware.
+const SetUserAgentMiddlewareName = "user-agent"
+
+// SetUserAgentMiddleware is middleware that sets the HTTP header User-Agent on the outgoing request.
+// If User-Agent already set, it will not be overridden by this middleware.
+func SetUserAgentMiddleware(userAgent string) httpclient.Middleware {
+	return httpclient.NamedMiddlewareFunc(SetUserAgentMiddlewareName, func(opts httpclient.Options, next http.RoundTripper) http.RoundTripper {
+		if userAgent == "" {
+			return next
+		}
+
+		return httpclient.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			if req.Header.Get("User-Agent") == "" {
+				req.Header.Set("User-Agent", userAgent)
+			}
+			return next.RoundTrip(req)
+		})
+	})
+}