[Feature] 对于doris on es 动态扩展字段的支持

[1sunhanya]

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Description

在使用doris on es的过程中，发现需要在doris中为es中的目标index建立外表，且外表中的字段都是固定的。而es自身在特定参数下是可以支持动态扩展mappin的。由此想询问是否可以在doris on es中支持动态扩展doris字段，以达到更好的操作es

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