Add a way to register the lower-case version of a conflicting MXID

[matrixbot]

This issue has been migrated from #15931.

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This isn't meant to merge to main-line and is just a one-off fix so I can register a specific user.

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Add a way to register the lower-case version of a conflicting MXID.

Basically, if you have @Foo:matrix.org already registered, this will allow you register @foo:matrix.org

poetry run register_new_matrix_user --inhibit_user_in_use_error 1 -c homeserver.yaml

Fix matrix-org/synapse#11426

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inhibit_user_in_use_error is already a homeserver configuration option but only applies to the username availability endpoint. This change makes it possible to configure the option when using the admin API to register a user.

Testing strategy

1. Add a conflicting user psql --user postgres synapse

   INSERT INTO users
       (name, password_hash, creation_ts, is_guest, admin, shadow_banned, approved)
   VALUES
       ('@test-COLLISION-user:my.synapse.linux.server', 'fake-password-hash', 1689216488, 0, 0, False, True);

2. poetry run register_new_matrix_user --inhibit_user_in_use_error 1 -c homeserver.yaml
   • Enter test-collision-user and whatever data for the rest
3. Notice the Success! message

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file.
• Pull request includes a sign off
• Code style is correct
  (run the linters)