Add WebApp management in firebase-admin/project-management

**Is your feature request related to a problem? Please describe.**
I want to totally create my Firebase infrastructure using code. Unfortunately, I cannot create web apps using firebase-admin/project-management; only Android and iOS apps are available.

**Describe the solution you'd like**
Methods in [ProjectManagement](https://firebase.google.com/docs/reference/admin/node/firebase-admin.project-management.projectmanagement) to handle WebApps, same as Android and iOS apps.

**Describe alternatives you've considered**
Extending the `ProjectManagement` object by myself, adding the methods after copying what's done for Android. But there are a couple of problems and it's not really efficient.

**Additional context**
None