shim: Move pprof server to plugin

Makes the pprof server a plugin and also gates by the `shim_tracing`
build tag (like otel is).
With this change, `net/http` is no longer a dependency in the shim.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/containerd-shim-runc-v2/main_tracing.go

@@ -18,4 +18,7 @@
 
 package main
 
-import _ "github.com/containerd/containerd/v2/pkg/tracing/plugin"
+import (
+	_ "github.com/containerd/containerd/v2/internal/pprof"
+	_ "github.com/containerd/containerd/v2/pkg/tracing/plugin"
+)