Add linkDomain support for ActionCodeSettting. This will generate Hosting-formatted email link.

[Xiaoshouzi-gh]

including error codes and sample app change

Hey there! So you want to contribute to a Firebase SDK?
Before you file this pull request, please read these guidelines:

Discussion

• Read the contribution guidelines (CONTRIBUTING.md).
• If this has been discussed in an issue, make sure to link to the issue here.
  If not, go file an issue about this before creating a pull request to discuss.

Testing

• Make sure all existing tests in the repository pass after your change.
• If you fixed a bug or added a feature, add a new test to cover your code.

API Changes

• At this time we cannot accept changes that affect the public API. If you'd like to help
  us make Firebase APIs better, please propose your change in a feature request so that we
  can discuss it together.

[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

Generated by 🚫 Danger

[paulb777]

Please add an update to the Changelog for the Release notes.

[paulb777]

Should this be marked deprecated? Or wait until next release?

[Xiaoshouzi-gh]

When I mark this as deprecated, our test failed due to build warnings. Any suggestions? I still want to keep the dynamicLinkDomain tests because both solutions will be available until FDL sunset.

[paulb777]

You can temporarily add --allow-warnings with a comment to the pod lib lint test invocations

[paulb777]

Closing since linkDomain was merged from #14384