/call endpoint

[patrick-ogrady]

Call invokes an arbitrary, network-specific procedure call with network-specific parameters. The guidance for what this endpoint should or could do is purposely left vague. In Ethereum, this could be used to invoke eth_call to implement an entire Rosetta API interface for some smart contract that is not parsed by the implementation creator (like a DEX). This endpoint could also be used to provide access to data that does not map to any Rosetta models instead of requiring an integrator to use some network-specific SDK and call some network-specific endpoint (like surfacing staking parameters).

Call is NOT a replacement for implementing Rosetta API endpoints or mapping network-specific data to Rosetta models. Rather, it enables developers to build additional Rosetta API interfaces for things they care about without introducing complexity into a base-level Rosetta implementation. Simply put, imagine that the average integrator will use layered Rosetta API implementations that each surfaces unique data.

Changes

• [x] add /call endpoint
• [x] add call_methods to Allow
• [x] add CallRequest
• [x] add CallResponse
• [x] Update README

[heimdall-asguard]

If Heimdall is stuck, please click the following link: https://heimdall-public.cbhq.net/v1/pull_request/stuck?pr_number=55&repository=coinbase/rosetta-specifications