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/provider.go

@@ -0,0 +1,32 @@
+package httpclient
+
+import (
+	"crypto/tls"
+	"net/http"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+)
+
+// Provider provides abilities to create http.Client, http.RoundTripper and tls.Config.
+type Provider interface {
+	// New creates a new http.Client given provided options.
+	New(opts ...httpclient.Options) (*http.Client, error)
+
+	// GetTransport creates a new http.RoundTripper given provided options.
+	GetTransport(opts ...httpclient.Options) (http.RoundTripper, error)
+
+	// GetTLSConfig creates a new tls.Config given provided options.
+	GetTLSConfig(opts ...httpclient.Options) (*tls.Config, error)
+}
+
+// NewProvider creates a new HTTP client provider.
+// Optionally provide ProviderOptions options that will be used as default if
+// not specified in Options argument to Provider.New, Provider.GetTransport and
+// Provider.GetTLSConfig.
+// If no middlewares are provided in opts the DefaultMiddlewares() will be used. If you
+// provide middlewares you have to manually add the DefaultMiddlewares() for it to be
+// enabled.
+// Note: Middlewares will be executed in the same order as provided.
+func NewProvider(opts ...httpclient.ProviderOptions) *httpclient.Provider {
+	return httpclient.NewProvider(opts...)
+}