plugin/cache: add a new keepttl option (#5879)

adds a new option `keepttl` to the cache plugin

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

plugin/cache/cache.go

@@ -48,6 +48,9 @@ type Cache struct {
 	pexcept []string
 	nexcept []string
 
+	// Keep ttl option
+	keepttl bool
+
 	// Testing.
 	now func() time.Time
 }