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/logging.go

@@ -117,7 +117,7 @@ func (h *LoggingHandler) Enabled(ctx context.Context, level slog.Level) bool {
 	// This is also checked in ServerSession.LoggingMessage, so checking it here
 	// is just an optimization that skips building the JSON.
 	h.ss.mu.Lock()
-	mcpLevel := h.ss.logLevel
+	mcpLevel := h.ss.state.LogLevel
 	h.ss.mu.Unlock()
 	return level >= mcpLevelToSlog(mcpLevel)
 }