Optional. Identifier of the conversation. Generally it's
auto generated by Google. Only set it if you cannot wait for
the response to return a auto-generated one to you.
The conversation ID must be compliant with the regression
fomula "`a-zA-Z][a-zA-Z0-9_-]`*" with the characters length
in range of [3,64]. If the field is provided, the caller is
1. the uniqueness of the ID, otherwise the request will be
2. the consistency for whether to use custom ID or not under
a project to better ensure uniqueness.