stackeval: An OpenTelemetry tracer

This just introduces an OpenTelemetry tracer in a package-level variable
but doesn't actually make use of it yet. We'll use it in subsequent
commits to help with debugging how different evaluation tasks interact
with one another.

internal/stacks/stackruntime/internal/stackeval/telemetry.go

@@ -0,0 +1,12 @@
+package stackeval
+
+import (
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/trace"
+)
+
+var telemetry trace.Tracer
+
+func init() {
+	telemetry = otel.Tracer("github.com/hashicorp/terraform/internal/stacks/stackruntime/internal/stackeval")
+}