Ban bugprone-forward-declaration-namespace which gives false-positives a lot

```
contrib/restricted/abseil-cpp/absl/base/internal/low_level_scheduling.h:37:7:
    error: no definition found for 'Mutex', but a definition with the same name 'Mutex'
    found in another namespace 'engine' [bugprone-forward-declaration-namespace,-warnings-as-errors]

   37 | class Mutex;
      |       ^
taxi/uservices/userver/core/include/userver/engine/mutex.hpp:28:7:
	note: a definition of 'Mutex' is found here

   28 | class Mutex final {
```

[Дока](https://clang.llvm.org/extra/clang-tidy/checks/bugprone/forward-declaration-namespace.html) также не показывает, почему это проблема.
[Греп](https://nda.ya.ru/t/zUk2T9GL7ekfWJ показывает, что почти все проверку отключают.
commit_hash:dfb4dda963bde4902387c980153bae9d2659d401

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