C API: Add rocksdb_memory_allocator_create_custom

[edmanlog123]

Summary
This PR adds rocksdb_memory_allocator_create_custom to the C API. This allows language bindings (Rust, Go, Python, etc.) to implement and plug in their own custom memory allocators. Currently, the C API only supports creating a jemalloc based allocator.

Motivation
This is a critical addition for improving RocksDB performance on Windows. The default Windows CRT allocator does not decommit memory to the OS, leading to unbounded memory growth in long-running processes (see #4112). By exposing this C-bridge, users can now plug in alternative allocators like mimalloc or tcmalloc via their respective language bindings to resolve these memory issues.

Changes
include/rocksdb/c.h: Added the rocksdb_memory_allocator_create_custom function signature and necessary callback typedefs.

db/c.cc: Implemented the CustomMemoryAllocator proxy class that inherits from rocksdb::MemoryAllocator and dispatches calls to the user-provided C function pointers.

db/c_test.c: Added a new test case CheckCustomAllocator to verify that the custom allocator is correctly called and that the DB opens/operates successfully with it.

Test Plan
Environment: macOS M2 (Apple Silicon).

Build: Verified with a static build using make c_test LIB_MODE=static.

Execution: Ran ./c_test and confirmed the CheckCustomAllocator test passed, ensuring the C-to-C++ bridge and lifecycle management (via shared_ptr) work as intended.

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