Introduce GraphQL endpoint

[adzialocha]

This PR just adds an GraphQL endpoint to the server, not doing anything yet.

• Refactor code-base to separate HTTP server functionality more clearly from JSON RPC
• Introduce async_graphql GraphQL framework
• Run an /graphql endpoint for POST (queries) and GET (playground) requests

Closes: #73

📋 Checklist

• Add tests that cover your changes
• Add this PR to the Unreleased section in CHANGELOG.md
• Link this PR to any issues it closes
• New files contain a SPDX license header

[sandreae]

Looks good to me! Thanks for getting the changeover to GraphQL started 👍

[cafca]

How do I use this? I cargo run and then open the playground at http://localhost:2020/graphql but there i get an error "Server cannot be reached". I thought I could do

{
  ping
}

?

[cafca]

I like it! I added a small docstring.

The problem I had above - if that was just some mistake I made this can be merged!