增加 eqSql 方法。

[Muyangmin]

该Pull Request关联的Issue

#6028

修改描述

参考 inSql 方法，在 Func、AbstractWrapper和 AbstractChainWrapper 中分别增加了对应的 eqSql 方法定义和实现。

[VampireAchao]

why not com.baomidou.mybatisplus.core.conditions.interfaces.Join#apply(java.lang.String, java.lang.Object...)?

[Muyangmin]

You are right, the apply method can do this. But as I mentioned in #6028 ,

1. Add a special method could reduce string templates and placeholders, which will improve code readability.
2. There is already inSql, leSql... series method, maybe this eqSql method is better match function semantic consistence?

Just for discussion :)