Move the contracts extensions to an external repository

[vplasencia]

Description:

There were two extension contracts in Semaphore v3: https://github.com/semaphore-protocol/semaphore/tree/v3.15.2/packages/contracts/contracts/extensions

The idea is to use the latest Semaphore v4 to update those contracts and add them to the Semaphore Extensions repo: https://github.com/semaphore-protocol/extensions