Which Bitcoin RPC call is used for setting transaction priority? Is there is any bitcoin RPC call for setting the transactions priority ?
The transaction priority is implied by the fee which you set for it. You can estimate the fee with your desired priority using https://bitcoincore.org/en/doc/0.16.0/rpc/util/estimatesmartfee/ 

Simply compute your transaction size (bytes) and adjust the transaction fee according to the sat/kB returned by the estimatesmartfee call.