Add chainHead_unstable_wasmQuery

Add a JSON-RPC function that allows you to pass a user-chosen Wasm program that has access to the chain storage and builds a return value.

The advantage is that this program could be sent over the network to a full node, which executes it, returns a proof, and sends the proof back to a light client, which then runs the program again to verify the proof.

Things that should be done:

• Add the networking protocol between the light and full node.
• Figure out how to let frontend JS developers easily create these Wasm programs. We probably don't want to ask them to setup a complicated Rust toolchain and let them figure out how to optimize the size of the Wasm program and all.

Neither of these two things strictly needs to be done before we add the JSON-RPC function.
A light client can emulate this function by executing the Wasm program locally and doing a storage query whenever a key is read by the program. This is not great but it works.

The issue names the chainHead_unstable_wasmQuery JSON-RPC entry point but no files or tests. Start by locating the JSON-RPC definitions and existing light/full-node interfaces, then trace how Wasm programs access chain storage. Done means the function’s execution and return behavior are specified and implemented; networking and frontend tooling are listed as follow-up work.