Adopting `with`- style accessors as part of the Transport APIs

[Joannis]

Is your feature request related to a problem? Please describe.
The current Transport APIs require the user to call connect() to establish a connection. From this point, the user has to manually track whether or not the connection is still alive, by handling any errors thrown by send or receive.

If a user wants to correctly handle error cases, this leads to more complexity in their implementations.

Describe the solution you'd like
In the Swift on Server ecosystem, we've adopted a with- style pattern, which compliments prior art in Swift's standard library. In there, we have a "bootstrap" for each transport that can be configured with the intended behavior of a transport connection.

let bootstrap = MCPTransportBootstrap()

A bootstrap connects in a with- style function call like so:

bootstrap.withClient { send, messages in
  try await send(SomeToolCall())

  for try await message in messages {
    // Handle incoming traffic
  }
}

Returning out of the withClient scope closes and cleans up the connection.
This pattern can be wrapped in more domain specific use cases like a request/response style flow in the higher level APIs.