feat: [MOB-121] Add LinkDomain to ActionCodeSettings

[graeme-verticalscope]

About this change

Add the LinkDomain field to the ActionCodeSettings struct, this new field enables migrating off of firebase dynamic links, which is deprecated and will be unsupported soon.

See issue firebase#681, and original PR firebase#682

Unit test is updated to check that linkDomain is set correctly in the action code settings map.

What should reviewers focus on

I've created a branch vs-patch-1-v4-15-2, to be the target branch for this change. I don't want to update the master branch because that will muddy our ability to keep the master branch in sync with the original repository.

Plan for maintaining our fork is:

• Sync master, and ensure the latest commit on master matches the commit id of the latest release tag in the original repository
• Make a patch branch off of the master branch, named something like <patch-name>-<release-version>
• Apply our patch commit on top of the branch
• Create a release tag like <release-version>-vspatch.1 (see go module versioning)
• Use the replace feature in go.mod to switch fl-core to our patched release version (see using forked package import in go)

[graeme-verticalscope]

For quick reference, here's how I created the tag. Run this on the the latest version of the patch branch to create your patch tag:

git tag -a v4.15.2-vspatch.1 -m "VerticalScope patch 1 of firebase-admin-go v4.15.2"
git push origin v4.15.2-vspatch.1