auth: suport ActionCodeSettings.linkDomain

[treitter]

This adds the ActionCodeSettings.linkDomain property to support custom domains in auth (overridding the default PROJECT_ID.firebaseapp.com authentication links).

This PR should fix issue 731.

Supported platforms:

• android
• ios
• js

Unsupported platforms:

• jvm (seems to be missing some lower-level support or I just missed the jvm dependency though it seems to just use the android one)

I haven't been able to test this since I haven't been able to resolve some issues with mavenLocal. But this is basically a copy-paste-edit of dynamicLinkDomain so it should work with, at most, trivial fixes.

[treitter]

I haven't been able to test this since I haven't been able to resolve some issues with mavenLocal. But this is basically a copy-paste-edit of dynamicLinkDomain so it should work with, at most, trivial fixes.

I worked out a solution for my mavenLocal issues and confirmed success accordingly:

• android: works
• ios: works
• js: untested but seems likely to work
• jvm: unimplemented (not sure the proper way to implement)

Please let me know if there's anything I can do to help get this merged and released.

Thanks again!