Use BCR cityhash module

[BYVoid]

Summary

• add the BCR cityhash@1.0.2.bcr.1 module dependency
• replace the Bazel-local vendored CityHash target with @cityhash//:cityhash
• remove the stale note that BCR cityhash is hash-incompatible; 1.0.2.bcr.1 now packages the frozen CityHash 1.0.2 source with the ssize_t compatibility patch

Why this can replace the Bazel part of #515

#515 fixes the Bazel-local :cityhash target under the assumption that Bazel keeps compiling contrib/cityhash. In that setup, the vendored city.cc included a generic config.h, so #515 renamed the checked-in header to city_config.h, prefixed its macros, and updated the local Bazel srcs list.

This PR removes that Bazel-local target entirely. Bazel no longer compiles contrib/cityhash; it consumes CityHash through the BCR module instead. That makes the Bazel-specific part of #515 unnecessary: there is no local :cityhash rule, no local city_config.h entry to maintain, and no local Windows /DWIN32 workaround for CityHash. The CMake/vendor-side cleanup from #515 can still be useful for the checked-in contrib copy, but Bazel no longer needs to own or patch that copy.

In short: #515 makes the vendored Bazel CityHash target safer; this PR stops vendoring CityHash in Bazel. The latter is a smaller maintenance surface for Bazel.

Why cityhash@1.0.2.bcr.1 matches the local contrib version

The local contrib/cityhash copy is based on upstream google/cityhash@bc38ef45ddbbe640e48db7b8ef65e80ea7f71298, i.e. CityHash 1.0.2. Later local changes were compatibility/build-system edits, not intentional hash algorithm changes.

The BCR cityhash@1.0.2.bcr.1 module points to that same upstream commit and applies the same relevant compatibility fix for ssize_t (remove_ssize_t.patch). That patch matches the local c02157e-style fix and preserves the CityHash64/CityHash128 behavior.

I also verified this locally by comparing outputs from patched BCR CityHash and the checked-in contrib CityHash for multiple input lengths across:

• CityHash64
• CityHash64WithSeed
• CityHash128

The outputs were identical, and the server-dependent e2e tests with compressed roundtrips passed against a local ClickHouse server.

Testing

• bazel --ignore_all_rc_files test @cityhash//:all --registry=https://bcr.bazel.build
• bazel --ignore_all_rc_files test //... --registry=https://bcr.bazel.build
• bazel --ignore_all_rc_files test //ut:e2e_tests --test_output=errors --cache_test_results=no --registry=https://bcr.bazel.build

Also tested earlier with --//:tls=no and --//:tls=openssl variants while validating the cityhash swap.