Hopper: Hydra payment channel between gateway and consumer

[mmahut]

This is the tracking issue for the feature of hydra micro-payment channels between the API consumer (customer) and the gateway (eventually platform).

Deliverable story

As an Blockfrost API end user, micro-payment channel will be opened with the Gateway to exchange tokens in exchange for API keys for a certain number of API requests.

For more information about this mode, look at the Pay-per-use API use case on Hydra's website.

[mmahut]

For this milestone, we will need a light hydra client that not require the full node on the customer side.

The hydra team is already working on the Blockchain chain layer support cardano-scaling/hydra#1305.

[michalrus]

Here's an implementation:

• Add Hydra micropayments (gateway↔end user) #451