CustomDiffConfig and CustomCheckConfig

[blampe]

We should allow the user to define custom config diff logic as well as CustomCheckConfig.

For diff, something like the below might work but is awkward. The resource controller defines Configure so it would be most natural for it to also handle DiffConfig. (This feels related to #159 because the controller doesn't currently have any type information about its configuration.)

diff --git a/infer/configuration.go b/infer/configuration.go
index 6fd15ac..6249f3f 100644
--- a/infer/configuration.go
+++ b/infer/configuration.go
@@ -62,6 +62,10 @@ type CustomConfigure interface {
        Configure(ctx p.Context) error
 }

+type CustomDiffConfig[T any] interface {
+       DiffConfig(ctx p.Context, olds T, news T) (p.DiffResponse, error)
+}
+
 type config[T any] struct{ t *T }

 func (*config[T]) underlyingType() reflect.Type {
@@ -144,6 +148,23 @@ func (c *config[T]) checkConfig(ctx p.Context, req p.CheckRequest) (p.CheckRespo

 func (c *config[T]) diffConfig(ctx p.Context, req p.DiffRequest) (p.DiffResponse, error) {
        c.ensure()
+
+       for _, ignoredChange := range req.IgnoreChanges {
+               req.News[ignoredChange] = req.Olds[ignoredChange]
+       }
+
+       var olds, news T
+       if _, err := ende.DecodeConfig(req.Olds, &olds); err != nil {
+               return p.DiffResponse{}, err
+       }
+       if _, err := ende.DecodeConfig(req.News, &news); err != nil {
+               return p.DiffResponse{}, err
+       }
+
+       if differ, ok := Config[T]().(CustomDiffConfig[T]); ok {
+               return differ.DiffConfig(ctx, olds, news)
+       }
+
        return diff[T, T, T](ctx, req, c.t, func(string) bool { return true })
 }

[iwahbe]

Both of these are possible by implementing infer.CustomCheck and infer.CustomDiff on the config struct.