Create a derivative user collection in hasadna to that in startup-os

[oferb]

startup-os has a user collection:
https://github.com/google/startup-os/blob/master/reviewer_config.prototxt

We'd like hasadna to have a derivative one. It means that a user can be:

• Defined only in startup-os
• Defined only in hasadna
• Defined in both, in which case we merge the data. We give precedence to startup-os: for fields like name and ID we will use startup-os, for fields like contributions we will merge the lists.

The code to merge the data and upload it to Firestore is here:
https://github.com/google/startup-os/blob/master/tools/reviewer/job/tasks/ReviewerMetadataUpdaterTask.java

So basically, this task is to update the code there to reflect the above.