This patch implements static cv::DFT function in RVV_HAL using native intrinsic, optimizing the performance for cv::dft and cv::dct with data types 32FC1/64FC1/32FC2/64FC2.

The reason I chose to create a new cv_hal_dftOcv interface is that if I were to use the existing interfaces (cv_hal_dftInit1D and cv_hal_dft1D), it would require handling and parsing the dft flags within HAL, as well as performing preprocessing operations such as handling unit roots. Since these operations are not performance hotspots and do not require optimization, reusing the existing interfaces would result in copying approximately 300 lines of code from core/src/dxt.cpp into HAL, which I believe is unnecessary.

Moreover, if I insert the new interface into static cv::DFT, both static cv::RealDFT and static cv::DCT can be optimized as well. The processing performed before and after calling static cv::DFT in these functions is also not a performance hotspot.

Tested on MUSE-PI (Spacemit X60) for both gcc 14.2 and clang 20.0.

$ opencv_test_core --gtest_filter="*DFT*"
$ opencv_perf_core --gtest_filter="*dft*:*dct*" --perf_min_samples=30 --perf_force_samples=30

The head of the perf table is shown below since the table is too long.

View the full perf table here: hal_rvv_dxt.pdf

Pull Request Readiness Checklist

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake