External id support

[PetrKubes97]

Hi, this is just a first draft with a bit more changes than necessary due to Flutter 3.44 warnings.

I can clean it up and send just the external_id change, if you want. :)

[claude]

Claude Code Review

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[DennisAlund]

Thanks @PetrKubes97! iOS side looks good.

Some change requests

1. Android actually supports external_id too — there's an 11-arg setUserData(...) overload with externalId since v18.1.x,
   inside our [18.0,19.0) range. Issue #1239 perhaps just left open and forgotten.
2. Drop the unrelated stuff. The Package.swift, plus the pubspec.lock bumps, .idea deletion, and test reformatting.
3. No release notes or version bumps, skip the CHANGELOG entry and any version changes, I'll handle those at release.

Just the external_id change with Android wired up like iOS. 🙏