Support multiple CdpClients with different CDP sessionId sharing a connection

[bwalderman]

We'll need a way to connect a CdpClient to a different CDP target than the browser target. In other words, support the "flat" mode of the protocol where multiple CDP sessions may share a connection and use the "sessionId" property to route messages to a particular session.

This is already used in mapperServer.ts to create and communicate with the mapper tab, and we will likely need it later to attach to related targets such as OOPIFs or workers.

Proposed API: Add a new method to CdpClient:
public attachToSession(sessionId: string): CdpClient

Given an existing CdpClient (e.g. the initial browser client), calling this method returns a new CdpClient that shares the same transport and can be used to send messages to or receive events from the given session.