Standard Relayer off-chain interface

[canepat]

As far as real adoption is concerned, the Relayer component has a key role in the Universal Login system.

The off-chain interface offered by the Relayer, referred as IF-6 in the system architecture diagram, could be IMHO well worth the proposal of a standard in terms of:

• the application layer format, i.e. JSON object matching ERC #1077 and #1078
• the transport layer format, i.e. Whisper protocol, HTTP protocol

This could also help the raising of multiple Relayer implementations, accessed indirectly via Whisper or directly via HTTP, which any client using the Universal Login SDK in Javascript (or another one in the language of choice) could force to compete against each other.

I would like to know if this sounds reasonable and if a PR documenting such standard interface (maybe in the Proposals repo?) would be appreciated.