add timestamp to custom-llm request messages payload

current messages array doesn't have timestamp, we want to it to have timestamp per each message so that we are able to insert system message to the right position

this is due to the fact that messages from vapi are dynamic, the message received from time 0 can be different from at time 1, in an non-incremental manner, meaning the old message can be override or completely tweaked by new messages.

having a timestame would be extremely beneficial for us to manage this dynamism, especially when we insert system message to the right spot of the message history