feat: add support for authenticatedUserToken

[jkaho]

What

• Add authenticatedUserToken field to event types
• Add setAuthenticatedUserToken method to enable users to set the field to be automatically passed in subsequent sendEvent calls
• Add getAuthenticatedUserToken method to enable users to get the authenticatedUserToken field
• Add onAuthenticatedUserTokenChange to enable passing a callback on authenticatedUserToken change
• Enable authenticatedUserToken to be passed on init

Links

EEX-746

More context:

• Insights user identity proposal (Confluence)
• Authenticated user token in Insights pipeline (Confluence)

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[francoischalifour]

In a similar manner to Segment's identify method I feel like we need a way to easily reset the identifiers. I know we're not at the point of adding an "identify" type event at the moment, but I just wanted to raise it.

Definitely! I wrote a proposal about accepting aa('setUserToken', undefined) (or aa('setAuthenticatedUserToken', undefined)) to restore the user token in RFC: Ability to restore anonymous user token in Search Insights.

Although it doesn't have to be implemented at the same time as this introduction, it's needed to support logout scenarios.

[jkaho]

@JasonBerry made some additional changes as per my commits 🙏