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/sigv4_middleware.go

@@ -0,0 +1,36 @@
+package httpclientprovider
+
+import (
+	"net/http"
+
+	"github.com/grafana/grafana-aws-sdk/pkg/sigv4"
+	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+)
+
+// SigV4MiddlewareName the middleware name used by SigV4Middleware.
+const SigV4MiddlewareName = "sigv4"
+
+var newSigV4Func = sigv4.New
+
+// SigV4Middleware applies AWS Signature Version 4 request signing for the outgoing request.
+func SigV4Middleware() httpclient.Middleware {
+	return httpclient.NamedMiddlewareFunc(SigV4MiddlewareName, func(opts httpclient.Options, next http.RoundTripper) http.RoundTripper {
+		if opts.SigV4 == nil {
+			return next
+		}
+
+		return newSigV4Func(
+			&sigv4.Config{
+				Service:       opts.SigV4.Service,
+				AccessKey:     opts.SigV4.AccessKey,
+				SecretKey:     opts.SigV4.SecretKey,
+				Region:        opts.SigV4.Region,
+				AssumeRoleARN: opts.SigV4.AssumeRoleARN,
+				AuthType:      opts.SigV4.AuthType,
+				ExternalID:    opts.SigV4.ExternalID,
+				Profile:       opts.SigV4.Profile,
+			},
+			next,
+		)
+	})
+}