feat: group data

[agazso]

This PR adds 2 new smart contracts:

• CypherCity has the application level functionality with functions such as joinGroup and addToGroup. It also has some helper functions (getMerkleRootCreationDate, getMerkleTreeRoot) that are either required for offchain proof verification or helps with optimizations. It interacts with the CypherCitySemaphore smart contract.
• CypherCitySemaphore: this is a thin wrapper over the Semaphore smart contract, exposing internal functionality.

Some notes:

• The joinGroup functionality is meant to be private, but I kept it public for now because it makes testing easier. Probably there is a pattern how to achieve this better.
• getMerkleRootCreationDate needs to be exposed so that the proofs of older messages can be verified offchain.
• getMerkleTreeRoot needs to be exposed so that it is cheap to check if the membership has changed.
• I spent a lot of time to make sensible tests, because they are actually integration tests and the order of the tests may change what is the environment they are executed in (they are non-deterministic). The simplest was to deploy a new set of contracts before each test runs and therefore make the environment more or less deterministic. The tradeoff is that this makes tests run much longer and with more tests added we may run out of the time allowed by the CI. Maybe there is a better way to do that. Also it would be good to omit the hardhat output.
• There is one test that is failing, but from the output it seems like it executes the test properly, which then results in a reverted transaction. However the error message (Error: cannot estimate gas) suggests that there is a problem with gas, but I think this is an environment or configuration issue.