Only invoke a given cached partial once

Note that this is backed by a LRU cache (which we soon shall see more usage of), so if you're a heavy user of cached partials it may be evicted and
refreshed if needed. But in most cases every partial is only invoked once.

This commit also adds a timeout (the global `timeout` config option) to make infinite recursion in partials
easier to reason about.

```
name              old time/op    new time/op    delta
IncludeCached-10    8.92ms ± 0%    8.48ms ± 1%   -4.87%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    6.65MB ± 0%    5.17MB ± 0%  -22.32%  (p=0.002 n=6+6)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      117k ± 0%       71k ± 0%  -39.44%  (p=0.002 n=6+6)
```

Closes #4086
Updates #9588

resources/images/config.go

@@ -19,7 +19,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/media"
 
 	"errors"
@@ -139,7 +139,7 @@ func DecodeConfig(m map[string]any) (ImagingConfig, error) {
 
 	i := ImagingConfig{
 		Cfg:     defaultImaging,
-		CfgHash: helpers.HashString(m),
+		CfgHash: identity.HashString(m),
 	}
 
 	if err := mapstructure.WeakDecode(m, &i.Cfg); err != nil {