PythonMutator: register product in user agent extra (#1533)

## Changes
Register user agent product following RFC 9110.

See
https://github.com/databricks/terraform-provider-databricks/pull/3520
for Terraform change.

## Tests
Unit tests

bundle/deploy/terraform/init.go

@@ -218,6 +218,23 @@ func setProxyEnvVars(ctx context.Context, environ map[string]string, b *bundle.B
 	return nil
 }
 
+func setUserAgentExtraEnvVar(environ map[string]string, b *bundle.Bundle) error {
+	var products []string
+
+	if experimental := b.Config.Experimental; experimental != nil {
+		if experimental.PyDABs.Enabled {
+			products = append(products, "databricks-pydabs/0.0.0")
+		}
+	}
+
+	userAgentExtra := strings.Join(products, " ")
+	if userAgentExtra != "" {
+		environ["DATABRICKS_USER_AGENT_EXTRA"] = userAgentExtra
+	}
+
+	return nil
+}
+
 func (m *initialize) Apply(ctx context.Context, b *bundle.Bundle) diag.Diagnostics {
 	tfConfig := b.Config.Bundle.Terraform
 	if tfConfig == nil {
@@ -262,6 +279,11 @@ func (m *initialize) Apply(ctx context.Context, b *bundle.Bundle) diag.Diagnosti
 		return diag.FromErr(err)
 	}
 
+	err = setUserAgentExtraEnvVar(environ, b)
+	if err != nil {
+		return diag.FromErr(err)
+	}
+
 	// Configure environment variables for auth for Terraform to use.
 	log.Debugf(ctx, "Environment variables for Terraform: %s", strings.Join(maps.Keys(environ), ", "))
 	err = tf.SetEnv(environ)

bundle/deploy/terraform/init_test.go

@@ -248,6 +248,27 @@ func TestSetProxyEnvVars(t *testing.T) {
 	assert.ElementsMatch(t, []string{"HTTP_PROXY", "HTTPS_PROXY", "NO_PROXY"}, maps.Keys(env))
 }
 
+func TestSetUserAgentExtraEnvVar(t *testing.T) {
+	b := &bundle.Bundle{
+		RootPath: t.TempDir(),
+		Config: config.Root{
+			Experimental: &config.Experimental{
+				PyDABs: config.PyDABs{
+					Enabled: true,
+				},
+			},
+		},
+	}
+
+	env := make(map[string]string, 0)
+	err := setUserAgentExtraEnvVar(env, b)
+
+	require.NoError(t, err)
+	assert.Equal(t, map[string]string{
+		"DATABRICKS_USER_AGENT_EXTRA": "databricks-pydabs/0.0.0",
+	}, env)
+}
+
 func TestInheritEnvVars(t *testing.T) {
 	env := map[string]string{}