Allow adding additional rules for one execution of protect without changing the client

[blaine-arcjet]

We want users to be able to vary rules given context they may only have in the handler, but we don't want them to construct a whole new SDK client during each request.

We should add another API, such as withRule() that allows the SDK to be augmented for one call to protect(). For example:

const aj = arcjet(...);

async function handler(req) {
  const decision = await aj.withRule(...someConfig).protect(req);
}