Add OTP Generators

[hzalaz]

Now you can generate OTP codes using Guardian SDK (HOTP or TOTP), for TOTP you can obtain the generator

let secret: Data
let totp = try Guardian.totp(secret: secret, algorithm: .sha1) // Default 6 digits

// or use more digits and a diff hmac alg
let totp = try Guardian.totp(secret: secret, algorithm: .sha256, digits: 8)

and then to generate the code

let code = totp.new() // using current time and 30s period

// or passing date and period
let code = totp.new(time: Date().timeIntervalSince1970, period: 45)