[firebase-admin] dynamic custom link suffix not added in email verification links in auth admin

* Operating System version: -
* Firebase SDK version: -
* Firebase Product: firebase-admin
* Node.js version: 15.9.0
* NPM version: 7.5.3

### [REQUIRED] Step 3:

#### Steps to reproduce:

link-suffix is not added in the generatePasswordResetLink and generateEmailVerificationLink of auth admin (firebase-admin/lib/auth) like https://firebase.google.com/docs/dynamic-links/custom-domains

it should be automatically added or ActionCodeSettings should include such param, for instance if you have example.com/link dynamic link, example.com/?link=... is generated instead of example.com/link/?link=...

#### Relevant Code:

```
const link: string = await admin.auth().generateEmailVerificationLink(someEmail, {url: '...', dynamicLinkDomain: '...'});
const link: string = await admin.auth().generatePasswordResetLink(someEmail, {url: '...', dynamicLinkDomain: '...'});
```