Implement Subscriptions

[Laincy]

Opening a draft PR mostly to give a status update and keep myself organized. This is pretty much all new code, a couple things had to happen to make subscriptions work.

Subscriptions are made up of 2 components, a contract and a subscription:

• Contract - An offer made by a service provider, which other wallets can agree to. This stores information such as the subscription period, price, and who is allowed to use the subscription.
• Subscription - Wallets that have agreed to a contract. Stores information such as the start date of the current term, the time it will lapse at, and the current state of the subscription.

Logic for lapsing subscriptions has been implemented and tested. There are absolutely better ways to go about implementing it, but until I see a performance reason to do it what we have will work fine.

Kromer Authorization

Authorization for the Kromer V1 API is done using sessions stored on the server. Session IDs are passed as bearer auth.

• Login - Pass private key in request body and it will return a session UUID. Currently, sessions expire after 1 hour.
• Logout - By connecting with an authorized session, the current session will be ended and the UUID will no longer work.
• Internal API for revoking active sessions. Just an idea, but could be useful if we run into another soak.

Contract API

• Create contracts
• List contracts, with filters for specific owners and their status.
• Get info on a specific contract by ID
• List subscribers to specific contract.
• Edit contract

Subscriber API

• Subscribe to contracts
• List subscribers, with filters for specific wallets and status.
• Cancel Subscription.

Websocket API

• Notify transaction listeners when subscription payment is made
• Notify contract listeners when subscription status is changed or renewed

Let me know what I've missed, I'm sure there are obvious API's that I've forgotten. Will continue to update here as I continue to flesh things out.