[ENG-1158] Default participant identifier to None and select 'id' as dict

[desaxce]

This pull request allows users to change the participant ID in a thread.

First issue came up with with client.thread() as thread and trying to change thread.participant_id because the upsert was looking at the wrong field.

Second issue comes from the to_dict method which attempts to read self.participant_identifier, that's why we need to initialize it to None.

• [x] Add test

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This change is 

[linear[bot]]

ENG-1158 Cookbook example: Evaluate conversation

Add cookbook on how to evaluate a discussion. Rating a conversation. Goal: from the beginning, conv is well supported Example cookbook: customer satisfaction evaluation