Add Parse Server option autoSignupOnLogin to automatically create a user on log-in if none exists

[mtrezza]

New Feature / Enhancement Checklist

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Current Limitation

If a user is created with auth data, there is a single linkWith request that logs in an existing user, or if needed automatically creates a new user. Using email and password requires two separate requests, for example a log-in attempt, and if no user was found, signing up, or vice-versa.

Feature / Enhancement Description

Add new Parse Server option autoSignupOnLogin that short-circuits the login endpoint to automatically create a user if none exists yet.

[parse-github-assistant]

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