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/page/site.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/identity"
 
 	"github.com/gohugoio/hugo/config"
 
@@ -75,6 +76,9 @@ type Site interface {
 
 	// Returns a map of all the data inside /data.
 	Data() map[string]any
+
+	// Returns the identity of this site.
+	GetIdentity() identity.Identity
 }
 
 // Sites represents an ordered list of sites (languages).
@@ -117,6 +121,10 @@ func (t testSite) Current() Site {
 	return t
 }
 
+func (t testSite) GetIdentity() identity.Identity {
+	return identity.KeyValueIdentity{Key: "site", Value: t.l.Lang}
+}
+
 func (t testSite) IsServer() bool {
 	return false
 }