Payment and trial scenarios to support

[shazow]

While I probably won't get around to implementing all of these myself, I'd like for the design of the code to support these scenarios for the future in a modular way:

Payment

• Smart contract like https://github.com/vipnode/vipnode-contract
• Credit card (e.g. Stripe)
• Out-of-band whitelisting (trusted API?)

Trial

• Time-based trial (e.g. 30min free per nodeID)
• Limited RPC proxy trial like Re-enable LES as option status-im/status-go#1025 (comment)