Oura multi-chain scaffold

[paulobressan]

What

• Oura will support bitcoin, ethereum and substrate.
• We need an initial scaffold of how to support these new features

Why

• because adding more chains expands the user-base of the tool
• because Oura is not particularly tied to Cardano, so there's no strict benefit on restricting it
• because we have a Catalyst proposal for doing this and the 1st milestone is the scaffold

DoD

• we have a new Source stage for Ethereum and Substrate sources (bitcoin will reuse utxorpc)
• the Event struct that holds data has different variants for each chain
• All of this changes are available in a single PR for easy review
• We submitted the Catalyst proof of achievement for MS1

Implementation details

• Each chain will be abstracted as a different "source" stage.
• Filters and Sink stages should remain the same
• The "Event" structure that gets passed through the pipeline should have different variants for each chain
• Bitcoin should be integrated via UTxO RPC
• Ethereum and Substrate should be integrated by calling their respective RPC interfaces