IndexOption的priority字段是小写，包外无法访问

[nowindexman]

我们现在有自己开发数据库驱动的需求，在实现index时候，发现priority小写，导致联合索引的排序在包外无法实现。
index相关的字段，除了priority其他所有字段都为大写，包外可访问，此字段是否改为大写

[nowindexman]

schema包下所有结构体，除了这个字段，其他都为大写

[github-actions]

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