[Add] Pixiv 添加一组与 Pixiv 登录会话相关的类;

[Add] PixivLogin, PixivLoginException 初版的登录器接口;
[Add] PixivSession, BasicPixivSession Pixiv 帐号登录会话抽象类和一个默认基本实现;
[Add] ObjectInputStreamLogin 通过反序列化 CacheStore 进行登录的登录器;

ContentGrabbingJi-pixiv/src/main/java/net/lamgc/pixiv/PixivSession.java

@@ -0,0 +1,43 @@
+package net.lamgc.pixiv;
+
+import org.apache.http.client.CookieStore;
+
+import java.io.IOException;
+import java.util.Objects;
+
+/**
+ * Pixiv 会话.
+ * @author LamGC
+ */
+public abstract class PixivSession {
+
+    private final CookieStore cookieStore;
+
+    protected PixivSession(CookieStore cookieStore) {
+        this.cookieStore = Objects.requireNonNull(cookieStore);
+    }
+
+    /**
+     * 登出当前会话.
+     *
+     * <p>登出后该会话 Cookies 将会失效.
+     * @return 如果登出成功, 返回 true.
+     * @throws IOException 当尝试登出发生异常时抛出.
+     */
+    public abstract boolean logOut() throws IOException;
+
+    /**
+     * 检查是否已登录, 或者说会话是否有效.
+     * @return 如果会话已登出(失效), 返回 false.
+     */
+    public abstract boolean isLogin();
+
+    /**
+     * 获取 CookieStore 对象.
+     * @return 返回 CookieStore.
+     */
+    public CookieStore getCookies() {
+        return cookieStore;
+    }
+
+}