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

### [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)
}
```