[Feature] doris cross-cluster insert

[TsukiokaKogane]

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Description

Following up on #57898, we plan to implement cross-cluster insert functionality based on the External Doris Catalog. This feature will enable read-write splitting for non-cloud Doris across independent clusters, delivering greater deployment flexibility. Compared with regular insert operations, we will reuse the majority of the existing implementation logic and develop new RPC methods to support transaction operations for remote FE.

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Are you willing to submit PR?

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