add admin slab release command

Summary:
Adds a new `release-ac-slabs` admin command to the ucache admin command handler that releases slabs from a specified allocation class back to the pool via CacheLib's `releaseSlab()` API.

**Usage:**
```
uc release-ac-slabs <class_id> [--count N]
```

**Arguments:**
- `class_id` — allocation class index (0-based)
- `--count N` — number of slabs to release (default: 1)

**Example:**
```
uc release-ac-slabs 5 --count 3
```
This releases 3 slabs from allocation class 5. Each slab release is timed and logged individually. If a release fails, the command stops and reports the error.

To make `CacheAllocator::releaseSlab()` accessible from ucache, `UcacheAdminCommandHandler` is added as a friend class in `CacheAllocator.h`, keeping the method private rather than exposing it publicly.

Reviewed By: AlnisM

Differential Revision: D100408757

fbshipit-source-id: 17217df0c54f4820e3f11102653b2755306cbe85

Author: IvanTopolcic

cachelib/allocator/CacheAllocator.h

@@ -1908,6 +1908,7 @@ class CacheAllocator : public CacheBase {
                                   .poolId = allocInfo.poolId});
   }
 
+ public:
   // Releases a slab from a pool into its corresponding memory pool
   // or back to the slab allocator, depending on SlabReleaseMode.
   //  SlabReleaseMode::kRebalance -> back to the pool
@@ -1959,6 +1960,7 @@ class CacheAllocator : public CacheBase {
                    SlabReleaseMode mode,
                    const void* hint = nullptr) final;
 
+ private:
   // @param releaseContext  slab release context
   void releaseSlabImpl(const SlabReleaseContext& releaseContext);