frontend: optionally pass region code 3rd party widgets

[thisconnect]

This may help Pocket in the future to pre-select users region in their widget.

Before asking for reviews, here is a check list of the most common things you might need to consider:

• updating the Changelog
• writing unit tests
• checking if your changes affect other coins or tokens in unintended ways
• testing on multiple environments (Qt, Android, ...)
• having an AI review your changes

[thisconnect]

cc @davidknezic

[davidknezic]

Hey @thisconnect, wonderful! We did not forget this PR and will keep you updated when we're ready from our side.