C-17237 Add EU endpoint configuration support to courier-flutter

[Gabrielgvl]

Summary

• add public CourierApiUrls presets for US and EU in the Flutter wrapper
• pass apiUrls through the Dart, iOS, and Android bridges for CourierClient and Courier.shared.signIn
• document the EU endpoint usage pattern in the README and auth/client docs

Testing

• git diff --check
• Unable to run flutter test locally because flutter and dart are not installed on this host

[Gabrielgvl]

Superseded by #25, which is the same branch pushed directly to the upstream repo so the PR workflows can run there normally.

[Gabrielgvl]

Closing in favor of #25.