Add support for master key clients to create user sessions (#7406)

* 6641: Implement support for user impersonation: master key clients can log in as any user, without access to the user's credentials, and without presuming the user already has a session

* reworded changelog

* rebuilt package lock

* fit test

* using lodash flatMap

* bump to node 12 for postgres test

* revert test fit

* add node version to postgres CI

* revert package-lock

Co-authored-by: gormanfletcher <git@gormanfletcher.com>
Co-authored-by: Manuel <5673677+mtrezza@users.noreply.github.com>

CHANGELOG.md

@@ -101,6 +101,8 @@ ___
 - EXPERIMENTAL: Added new page router with placeholder rendering and localization of custom and feature pages such as password reset and email verification (Manuel Trezza) [#7128](https://github.com/parse-community/parse-server/pull/7128)
 - EXPERIMENTAL: Added custom routes to easily customize flows for password reset, email verification or build entirely new flows (Manuel Trezza) [#7231](https://github.com/parse-community/parse-server/pull/7231)
 - Added Deprecation Policy to govern the introduction of braking changes in a phased pattern that is more predictable for developers (Manuel Trezza) [#7199](https://github.com/parse-community/parse-server/pull/7199)
+- Add REST API endpoint `/loginAs` to create session of any user with master key; allows to impersonate another user. (GormanFletcher) [#7406](https://github.com/parse-community/parse-server/pull/7406)
+
 ### Other Changes
 - Fix error when a not yet inserted job is updated (Antonio Davi Macedo Coelho de Castro) [#7196](https://github.com/parse-community/parse-server/pull/7196)
 - request.context for afterFind triggers (dblythy) [#7078](https://github.com/parse-community/parse-server/pull/7078)