rocksdbconfig.cmake - improve locating zstd packages

[Andrej730]

Original problem I've met - on Windows configuring project that has find_package(RocksDB REQUIRED CONFIG) with

cmake .. -DCMAKE_PREFIX_PATH=L:\Software\usr\rocksdb-9.11.2;L:\Software\usr\zstd-1.5.7

resulted in

CMake Error at L:/Software/Microsoft Visual Studio/2022/Community/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.31/Modules/FindPackageHandleStandardArgs.cmake:233 (message):
  Could NOT find zstd (missing: ZSTD_LIBRARIES)
Call Stack (most recent call first):
  L:/Software/Microsoft Visual Studio/2022/Community/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.31/Modules/FindPackageHandleStandardArgs.cmake:603 (_FPHSA_FAILURE_MESSAGE)
  L:/Software/usr/rocksdb-9.11.2/lib/cmake/rocksdb/modules/Findzstd.cmake:17 (find_package_handle_standard_args)
  L:/Software/Microsoft Visual Studio/2022/Community/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.31/Modules/CMakeFindDependencyMacro.cmake:78 (find_package)
  L:/Software/usr/rocksdb-9.11.2/lib/cmake/rocksdb/RocksDBConfig.cmake:66 (find_dependency)
  CMakeLists.txt:6 (find_package)

Investigating I've found two issues and fixed in this PR:

• static library on zstd is using different name - zstd_static, which wasn't considered in Findzstd.cmake.
  https://github.com/facebook/zstd/blob/d190b3886305f0fb894cd648da6a6d9ecda56f64/build/cmake/lib/CMakeLists.txt#L197-L202
• zstd config packages (meaning zstd that has zstdconfig.cmake to locate headers and libraries automatically) were not supported. Since it's more modern way to include packages in cmake, it makes sense to check for them first and then fallback to good old modules.
  I've used find_package(zstd CONFIG QUIET), unlike i.e find_dependency(Snappy CONFIG) used for other dependencies, otherwise if RocksDB is REQUIRED then REQUIRED status is propagated by find_dependency and it would fail on trying to locate CONFIG and would never get to module.

@xingbowang can you please take a look?

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[Andrej730]

Signed CLA, seems to be pending.

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