Add YT_DEFINE_SENTINEL_OPTIONAL macro

Add a macro for defining `TSentinelOptional` type aliases when the value type is not structural (e.g. has protected members) and therefore cannot use `TValueSentinel<V>`. The macro collapses the boilerplate sentinel struct plus `using` declaration into a single line:

```cpp
// before
struct TInstantSentinel
{
    static constexpr auto Sentinel = TInstant::Zero();
};
using TSentinelOptionalInstant = TSentinelOptional<TInstant, TInstantSentinel>;

// after
YT_DEFINE_SENTINEL_OPTIONAL(TSentinelOptionalInstant, TInstant, TInstant::Zero());
```

Also convert the existing call sites in `service_detail.cpp` and `inferrum/block_cache.cpp`.
commit_hash:5dcdeb8db215736b0ce5a5b71f30aead91c7b8e8

0 files changed, 0 insertions, 0 deletions