feat: encode narrow RPC methods in beacon, propagate to messages

[harrysolovay]

Closes #108. This PR changes the RPC interface.

We must now create a beacon, the type of which is encoded with supported methods.

The following is a beacon that only supports the state_getMetadata method.

import * as rpc from "capi/rpc";

const beacon = rpc.beacon<"state_getMetadata">([URL]);

We can allow all methods by simply omitting the type arg.

const beacon = rpc.beacon([URL]); // supports all methods

Now we can create the client:

const client = await rpc.client(beacon);

We'll get a type error if we attempt to utilize methods that are NOT encoded in the beacon as accessible.

// Allow only the `state_getStorage` method
const client = await rpc.client(rpc.beacon<"state_getStorage">(polkadotBeacon));

// Ensure that the client was successfully opened
assert(!(client instanceof Error));

// Attempt to call a different method
const result = await client.call("state_getMetadata", []);
//                                ~~~~~~~~~~~~~~~~~
//                                ^
//                                Argument of type '"state_getMetadata"' is not assignable to parameter of type '"state_getStorage"'.
                                  

Ultimately, we'll want to package chain-specific beacons in known in order to protect developers from utilizing methods that don't exist.