add` book_changes` support for `subscribe` RPC

[shawnxie999]

currently xrpl.js doesn't seem to support book_changes. Ideally, book_changes can be used by the following code

client.request({
    command: "subscribe",
    streams: ["book_changes"],
  });
  client.on("bookChanges", async (x) => {
    console.log(x);
  });

[ckeshava]

Hello!

I'd like to add that both the book_changes JSON-RPC method and the book_changes subscription stream support is missing in the xrpl.js client library.

For the sake of feature-parity with rippled, both these need to be implemented in xrpl.js. These two methods are important in the context of the recently proposed PermissionedDEX XLS-81d amendment.

@shawnxie999 Thanks for raising this issue.