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/sse_test.go

@@ -49,7 +49,7 @@ func TestSSEServer(t *testing.T) {
 			})
 
 			c := NewClient(testImpl, nil)
-			cs, err := c.Connect(ctx, clientTransport)
+			cs, err := c.Connect(ctx, clientTransport, nil)
 			if err != nil {
 				t.Fatal(err)
 			}