mcp: support stateless streamable sessions (#277)

Support stateless streamable sessions by adding a GetSessionID function
to StreamableHTTPOptions. If GetSessionID returns "", the session is
stateless, and no validation is performed. This is implemented by
providing the session a trivial initialization state.

To implement this, some parts of #232 (distributed sessions) are copied
over, since they add an API for creating an already-initialized session.

In total, the following new API is added:
- StreamableHTTPOptions.GetSessionID
- ServerSessionOptions (a new parameter to Server.Connect)
- ServerSessionState
- ClientSessionOptions (a new parameter to Client.Connect, for symmetry)

For #10

mcp/session.go

@@ -0,0 +1,29 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp
+
+// hasSessionID is the interface which, if implemented by connections, informs
+// the session about their session ID.
+//
+// TODO(rfindley): remove SessionID methods from connections, when it doesn't
+// make sense. Or remove it from the Sessions entirely: why does it even need
+// to be exposed?
+type hasSessionID interface {
+	SessionID() string
+}
+
+// ServerSessionState is the state of a session.
+type ServerSessionState struct {
+	// InitializeParams are the parameters from 'initialize'.
+	InitializeParams *InitializeParams `json:"initializeParams"`
+
+	// InitializedParams are the parameters from 'notifications/initialized'.
+	InitializedParams *InitializedParams `json:"initializedParams"`
+
+	// LogLevel is the logging level for the session.
+	LogLevel LoggingLevel `json:"logLevel"`
+
+	// TODO: resource subscriptions
+}