Changes

feat(pipelines/pingcap/tiflash): add support for next-gen branch integration tests (#3539)

Close https://github.com/PingCAP-QE/ci/issues/3544

This commit introduces logic to handle the 'feature/next-gen' branch in
the integration test pipeline.
* It adds new variables to manage next-gen specific configurations
* modifies cache checks for proxy and libclara
* and updates the CMake command to include a flag for enabling next-gen
features.
* Additionally, it adjusts the TiKV image tag based on the branch type,
enhancing the CI process for next-gen development.

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Co-authored-by: JaySon <tshent@qq.com>
 (commit: fbb0299)