Plugins: add http cache duration

Author: naltatis

docs/devices/plugins.mdx

@@ -356,6 +356,7 @@ auth: # basic authentication
 insecure: false # set to true to trust self-signed certificates
 jq: .data.tuples[0][1] # parse response json
 scale: 0.001 # factor applied to result, e.g. for kW to W conversion
+cache: 60s   # response cache duration
 timeout: 10s # timeout in golang duration format,
              # see https://golang.org/pkg/time/#ParseDuration
 ```
@@ -534,4 +535,4 @@ plugged:
 charging:
   source: mqtt
   topic: openWB/lp/1/boolChargeStat
-```
+```