chore: refactor time.Duration -> int64 milliseconds for FE consumption (#1944)

* Changes all public-facing codersdk types to use a plain int64 (milliseconds) instead of time.Duration.
* Makes autostart_schedule a *string as it may not be present.
* Adds a utils/ptr package with some useful methods.

coderd/util/ptr/ptr.go

@@ -0,0 +1,23 @@
+// Package ptr contains some utility methods related to pointers.
+package ptr
+
+import "golang.org/x/exp/constraints"
+
+type number interface {
+	constraints.Integer | constraints.Float
+}
+
+// Ref returns a reference to v.
+func Ref[T any](v T) *T {
+	return &v
+}
+
+// NilOrEmpty returns true if s is nil or the empty string.
+func NilOrEmpty(s *string) bool {
+	return s == nil || *s == ""
+}
+
+// NilOrZero returns true if v is nil or 0.
+func NilOrZero[T number](v *T) bool {
+	return v == nil || *v == 0
+}