The Custody Keyring Class is the initial point of contact with a custodian, it gets access to the sdk and all methods of interaction with a custodian, like custodian accounts, sign a tx, account details, tx details.
[json-rpc-engine](https://github.com/MetaMask/json-rpc-engine) middleware implementing ethereum filter methods. Backed by an [eth-block-tracker](https://github.com/MetaMask/eth-block-tracker) and web3 provider interface (`web3.currentProvider`).