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

Start with the ProjectManagement API and inspect how its existing Android and iOS app methods are exposed. Add equivalent methods for managing WebApps, with behavior and coverage matching the existing mobile-app management surface; the issue does not name specific files or tests.

Written by the indexing model from the issue text.