chore: fix 30% startup time hit from userpassword (#12769)

pbkdf2 is too expensive to run in init, so this change makes it load
lazily. I introduced a lazy package that I hope to use more in my
`GODEBUG=inittrace=1` adventure.


Benchmark results:

```
$ hyperfine "coder --help" "coder-new --help"
Benchmark 1: coder --help
  Time (mean ± σ):      82.1 ms ±   3.8 ms    [User: 93.3 ms, System: 30.4 ms]
  Range (min … max):    72.2 ms …  90.7 ms    35 runs
 
Benchmark 2: coder-new --help
  Time (mean ± σ):      52.0 ms ±   4.3 ms    [User: 62.4 ms, System: 30.8 ms]
  Range (min … max):    41.9 ms …  62.2 ms    52 runs
 
Summary
  coder-new --help ran
    1.58 ± 0.15 times faster than coder --help
```

coderd/util/lazy/value.go

@@ -0,0 +1,28 @@
+// Package lazy provides a lazy value implementation.
+// It's useful especially in global variable initialization to avoid
+// slowing down the program startup time.
+package lazy
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+type Value[T any] struct {
+	once   sync.Once
+	fn     func() T
+	cached atomic.Pointer[T]
+}
+
+func (v *Value[T]) Load() T {
+	v.once.Do(func() {
+		vv := v.fn()
+		v.cached.Store(&vv)
+	})
+	return *v.cached.Load()
+}
+
+// New creates a new lazy value with the given load function.
+func New[T any](fn func() T) *Value[T] {
+	return &Value[T]{fn: fn}
+}