We need a way for the client to request configuration from an embedding
frame over `postMessage`.

JSON-RPC is a well-known, simple and convenient format for RPC messages.

There are several npm packages which claim to offer this functionality
but at the time of writing, none appeared to be sufficiently well
tested/supported/documented.

There is an existing partial postMessage JSON-RPC server implementation in
src/sidebar/cross-origin-rpc.js. The non-app specific parts of that will
be moved into this module in future.