Authorization Envelope and Payment Envelope

@kimcodeashian The private key for my payer for transactions rests on my backend. Basically I am building something where I could fund all the transactions happening on my platforms.

Blocto will only help with the authorizations of the user that is currently loggedin.

Anyways implemented my custom signing function following this.
It worked for my usecase. Let me know if you have any better suggestions though.