[FR] adding toJSON property to the Admin SDK Timestamp class

**Is your feature request related to a problem? Please describe.**
Working in monorepos or other situations with shared Typescript interfaces is complicated because toJSON property is missing on the Admin SDK Timestamp definition

Can firebase-admin package and firebase package share same type definition of some common class?
For example firebase-admin Timestamp class is defined [here](https://cloud.google.com/nodejs/docs/reference/firestore/latest/firestore/timestamp) but firebase Timestamp class is defined [here](https://firebase.google.com/docs/reference/node/firebase.firestore.Timestamp) and they are different.

**Describe the solution you'd like**
adding toJSON property to the Admin SDK Timestamp class to make it compatible to firebase Timestamp class.

**Describe alternatives you've considered**
Split the common class type definition to a separate package?

**Additional context**
Discussed at #1404 before, but no contributor response after [hiranya911](https://github.com/hiranya911) is not active in firebase project.