Added support for `credentials_url`, falling back to `server_url`, to be
used as the key for storing information inside Keychain Access. This
allows two copies of Zed to be used side by side if the other is
launched with a custom `--user-data-dir`.

- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects

Release Notes:

- Added support for `credentials_url`, falling back to `server_url`, to
be used as the key for storing information inside Keychain Access.