add eth_subscribe and eth_unsubscribe methods

[bomanaps]

re-submission of @Shiti PR #331 on adding eth_subscribe / eth_unsubscribe; submitted as documentation-only since the speccheck framework can't yet validate subscription methods, deferring that testing-framework decision to core devs.

[bomanaps]

@fjl on the subscription-ID problem you raised, would it work to extend the .io format with a capture token (something like "result": "" on subscribe, then $subId on unsubscribe), or does speccheck really need WebSocket support landed first before any of this is testable?

cc @MysticRyuujin

[fjl]

We have to upgrade the spec's OpenRPC version to a newer one to be able to specify notifications.

[bomanaps]

We have to upgrade the spec's OpenRPC version to a newer one to be able to specify notifications.

I have a question do you want eth_subscribe written as a pure notification method no result, or kept as a normal request/response that returns the subscription ID with a separate notification method carrying the pushed updates? and on the version itself should we target OpenRPC 1.3.x the latest open-rpc/meta-schema Go binding, v1.14.9? we can't do 1.4. it has no Go binding yet

[bomanaps]

closing this #797