fix: add setter for FastjsonConfig.writer/readerContext, for issue #4000

[jujn]

What this PR does / why we need it?

允许用户像 1.x 中那样限定自定义序列化器影响范围，如下所示：

        FastJsonConfig config = new FastJsonConfig();
        
        ObjectWriterProvider provider = new ObjectWriterProvider();
        provider.register(Long.class, ObjectWriters.ofToString(Object::toString));
        provider.register(Integer.class, ObjectWriters.ofToString(Object::toString));
        
        config.setWriterContext(JSONFactory.createWriteContext(provider));

Summary of your change

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[jujn]

done

[wenshao]

[Suggestion] 新增的 setReaderContext() / setWriterContext() 缺少测试覆盖和 Javadoc。

1. 测试缺失： PR 新增了 4 个公共方法，但 diff 中没有测试文件变更。建议在 FastJsonConfigTest 中补充：(a) setter/getter 往返一致性，(b) PR 描述中的自定义 ObjectWriterProvider 用例，(c) 废弃方法委托正确性，(d) clearContext() 后自定义 context 被清除的行为。

2. 缓存失效行为未文档化： 这两个 setter 注入的自定义 context 会被后续 setReaderFeatures()、setDateFormat() 等方法通过 clearContext() 静默清除。JavaBean 的 set/get 命名隐含"值会持续生效"的契约，建议添加 Javadoc 警告此行为：

/**
 * Sets a pre-built JSONReader.Context.
 * <p><b>Warning:</b> subsequent calls to {@link #setReaderFeatures},
 * {@link #setDateFormat}, etc. will invalidate this custom context.
 */
public void setReaderContext(JSONReader.Context context) {
    this.readerContext = context;
}

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