Add ActionCodeSettings.linkDomain

[graeme-verticalscope]

The ActionCodeSettings struct is missing the linkDomain field.

The linkDomain field is required to migrate off of dynamic links, it's part of the migration guide for using a custom Firebase Hosting domain

[google-oss-bot]

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[Xiaoshouzi-gh]

Hi @graeme-verticalscope do you want to create a pull request with cherry-pick against master? We can review it soon.

[graeme-verticalscope]

Hi @graeme-verticalscope do you want to create a pull request with cherry-pick against master? We can review it soon.

Thanks, I created a new PR with the commit cherry-picked on top of master: #703