FR: Allow for sending of an out-of-band email action link

[Dal-Papa]

[READ] Step 1: Are you in the right place?

✅

[REQUIRED] Step 2: Describe your environment

• Operating System version: Google App Engine, Go 1.16 Runtime
• Firebase SDK version: ?
• Library version: v4.8.0
• Firebase Product: auth

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

When using the EmailSignInLink or other functions that use generateEmailActionLink in the background, no email is sent despite the capability of the SDK to do so.

Relevant Code:

link, err := s.Client.EmailSignInLink(context.Background(), testEmail, testActionCodeSettingsNoReturn)
if err != nil {
	t.Fatal(err)
}