v1.5.1 add token faucet function [BAC-2018]

[jiajames]

No description provided.

[linear]

BAC-2018 Add a testnet (ropsten) token faucet

This can be done by adding a new endpoint on librarian that creates a layer-2 transfer from an existing account (controlled by dYdX) to the users account. This allows users to receive testnet tokens without us having to send a layer 1 transaction that could quickly drain our Ropsten wallets of ETH. This also makes it so that users don't have to have testnet ETH to start playing around in the testnet.

Endpoint details:

• add a "testnet" controller to librarian
• add a single private POST endpoint tokens (full path is v3/testnet/tokens )
• only active when config.isStaging() === true
• takes in no arguments
• transfer 10,000 (configurable in config) tokens to the user's account from the deposit-matcher account on staging

Extensions:

• Can start with using the lowTrafficRateLimiter, but should ultimately have its own rate-limiter (start with 5 times per day per IP address)

Other notes (mostly product notes):

• We still should still try and get users to on-chain register their starkKey with their ethereumAddress since otherwise they won't be able to complete slow withdrawals. This does request them to have ropsten ETH and we would have to point them to faucets such as here or here

[BrendanChou]

throw error if networkId is not 3?