resources: Add key to reources.GetRemote options map

If set, `key` will be used as the only cache key element for the resource.

The default behaviour is to calculate the key based on the URL and all the options.

This means that you can now do:

```
{{ $cacheKey := print $url (now.Format "2006-01-02") }}
{{ $resource := resource.GetRemote $url (dict "key" $cacheKey) }}
```

Fixes #9755

common/maps/maps.go

@@ -109,6 +109,20 @@ func ToSliceStringMap(in any) ([]map[string]any, error) {
 	}
 }
 
+// LookupEqualFold finds key in m with case insensitive equality checks.
+func LookupEqualFold[T any | string](m map[string]T, key string) (T, bool) {
+	if v, found := m[key]; found {
+		return v, true
+	}
+	for k, v := range m {
+		if strings.EqualFold(k, key) {
+			return v, true
+		}
+	}
+	var s T
+	return s, false
+}
+
 type keyRename struct {
 	pattern glob.Glob
 	newKey  string