Implement Shutter's API Service

[ylembachar]

See EPIC #1 for the full specification

This service is a helper service that allows to use the Shutter system:

• Has access to the keyper endpoint to retrieve old decryption keys
• Exposes the below endpoints:

Endpoint 1: Register Identity with Timestamp

• Allows a client to register an identity and time-based decryption trigger.
• Stores the client's identity and timestamp on the registry contract.
• Returns the encryption key for that identity.
• If the identity is not supplied, it returns a randomized identity together with the encryption key.

Endpoint 2: Get Encryption Key

• Allows a client to retrieve the encryption key for a certain identity.

Endpoint 3: Get Decryption Key

• Retrieve the decryption key once the timestamp is reached.
• Provides the decryption key to the client for local decryption.

Endpoint 4: Decrypt Commitments

• Perform decryption of encrypted commitments.
• Accepts encrypted data with an identity. Decryption keys are stored when released per identity.
• Outputs the decrypted data.

The service needs to offer the below additional functionalities:

• Stores decryption keys when released