Add persistentId field in Message for common attribute

[saipreetham16]

Issue Number: #55

Description:

What are the changes? Why are we making them?
This change introduces a new field called persistentId to the Message object. The persistentId can be used by customers to consistently track messages, as it will remain the same during both message initiation and creation.

Functional backward compatibility:

Does this change introduce backwards incompatible changes? [YES/NO]
YES

Does this change introduce any new dependency? [YES/NO]
NO

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Testing:

Is the code unit tested?
Yes this code i s tested locally
Have you tested the changes with a sample UI (e.g. Android Mobile Chat Example)?
No

List manual testing steps:

• Add Steps below:
• Initiated message creation with persistentId set to the same value as message Id.
• Verified that transcript messages retain the same persistentId during message creation

Here are a list of manual test cases to run through:

• Initiating chat and connecting with an agent
• Retrieving transcript
• Disconnecting from chat
• Sending a message to the agent
  • See typing bubbles on agent side
  • See read/delivered receipt on client side
  • Receiving a message from the agent
  • See typing bubbles on client side
  • See read/delivered receipt on agent side
  • Sending an attachment to the agent (try .txt, .pdf, .jpg)
  • Preview the attachment on click
  • Receiving an attachment from the agent
  • Preview the attachment on click
• Close the application (Without ending chat) → open app again → Start chat → Should Retrieve transcript from a previous chat session

[haomingli2020]

nit: we always want this to be triggered when Id is updated right? Can we just do it as part of updateId() function above instead of called separately?