Shape an interface for API providers for Tx building

[mpizenberg]

We will need API providers for different things in elm-cardano. The most obvious one is for Tx building. Finalization of transaction intents need local access to all relevant UTxOs. This means they must be preloaded before building the Tx. If that Tx only interacts with the wallet UTxOs, or with UTxOs we have created ourselves it is not a problem. But if we want to interact with UTxOs out there, we need a way to ask them.

It would be great to have a shared interface, defined as a record containing functions, that can be implemented by multiple API providers, such as Blockfrost, Koios, Kupmios, UTxORPC, etc.

Finding an interface that is convenient and compatible with most providers will require trial and errors with the respective ones. So I definitely need help here from people who are familiar with them.