Add PPCG block subspace solver

[Silver-Moon-Over-Snow]

Summary

• add the PPCG diagonalization solver and wire it into the PW hsolver path
• split PPCG implementation details into focused headers under source/source_hsolver/ppcg
• use ATen/container LAPACK kernels and ModuleBase vector dot kernels where applicable
• make the block-subspace strategy the PPCG production path and add PPCG coverage

Tests

• ctest --test-dir build-ppcg-openmp -R MODULE_HSOLVER_ppcg --output-on-failure
• ctest --test-dir build-ppcg-openmp -R MODULE_HSOLVER_pw --output-on-failure
• OMP_NUM_THREADS=4 ./build-ppcg-openmp/source/source_hsolver/test/MODULE_HSOLVER_ppcg --gtest_also_run_disabled_tests --gtest_filter=DiagoPPCGBenchmarkTest.DISABLED_FullBenchmark --gtest_brief=1

[Silver-Moon-Over-Snow]

Opened a new PR by mistake, I shall return to the PR #7580 and close this inappropriate PR.