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

[tlserver]

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 but firebase Timestamp class is defined here 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 is not active in firebase project.

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