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.