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/setup.go

@@ -240,6 +240,11 @@ func cacheParse(c *caddy.Controller) (*Cache, error) {
 				default:
 					return nil, fmt.Errorf("cache type for disable must be %q or %q", Success, Denial)
 				}
+			case "keepttl":
+				if len(args) != 0 {
+					return nil, c.ArgErr()
+				}
+				ca.keepttl = true
 			default:
 				return nil, c.ArgErr()
 			}