Update Client.connect(transport:) to automatically send initialize request

[mattt]

See #97 (comment)

Other SDKs have clients automatically send initialize message when connecting to a server. This PR updates our implementation to do the same. Beyond consistency, this helps mitigate an invisible time dependency / race condition for clients with streamable HTTP transport, as the client waits for a session ID before subscribing to SSE events. If the API consumer failed to call initialize immediately after connecting, this could result in SSE subscriptions timing out.

This PR deprecates the existing Client.initialize method. This method will be removed in a future version (or more accurately, it will be made private and replace the current private func _initialize method).