Add script to be able to dump user with backing identity information

As part of the sunset plan for Gitter, we need something where people can use GitLab/GitHub/Twitter social login on Element against the gitter.im (gitter.ems.host) homeserver and land with all of their same membership that they natively had on Gitter.

@clokep laid out a great plausible plan to make this happen. To summarize that plan:

Synapse has an user_external_ids table that it uses to match the remote ID from the backing identity to the MXID. So what we need is to dump the information from the Gitter database, massage it a bit and inject it into the Synapse homeserver database.

This issue is tracking the script to do the data dumping from the Gitter MongoDB database.

Definition of done

• Script that streams out the following information for all users in the database and persists to a local data dump file
  • matrixId and Gitter userId from MatrixBridgedUserSchema
  • provider and providerKey from IdentitySchema
  • Add resume position option so we can make an incremental dump once we disable user creation on Gitter and move over the final people during the cut-over