Add canSendMessage state & update ChatHeadless constructor

[yen-tt]

• added canSendMessage state -- true when there's no message currently being process and false otherwise
  • when it's false, it will prevent getNextMessage and streamNextMessage from sending a call to API as another message is still being processed
• updated loading state such that it's set to false as soon as the response starts instead of when a response is fully completed
• updated ChatHeadless constructor by grouping the params into a HeadlessConfig interface.
  • in a separate PR, I will update chat-headless-react to use HeadlessConfig. This way, future changes to headless's constructor -- such as adding new fields -- will not need code change in chat-headless-react

J=CLIP-158
TEST=manual&auto

see the updated jest test passed
see that the expected behavior works in test-site (loading dots controlled by loading state and input enable/disable controlled by canSendMessage)