[Feat] Support the joined clusters can be set OFFLINE

[xueliangyang-oeuler]

OFFLINE means that all the requests come to the offline clusters will be bypass and return normally, and

• vela-core can recognize the defined return code, and set the status for applicaiton cr depending on the defined return code
• vela-apiserver can recognize the defined return code, and return the Rest-API or CLI call depending on the defined return code