C API: Add rocksdb_memory_allocator_create for custom allocators

[varun29ankuS]

Summary

Add a callback-based rocksdb_memory_allocator_create() to the C API, enabling
language bindings to plug in custom memory allocators (mimalloc, tcmalloc, snmalloc)
at runtime without requiring them to be linked at RocksDB build time.

Resolves #14367

Motivation

The C API currently only exposes rocksdb_jemalloc_nodump_allocator_create().
On Windows (MSVC), jemalloc is not supported, and the CRT allocator never
decommits freed heap segments — causing unbounded memory growth (#4112, #12364).
Language bindings (Rust, Go, Python) have no way to plug in an alternative.

Implementation

Follows the exact callback pattern used by rocksdb_comparator_create,
rocksdb_mergeoperator_create, and rocksdb_compactionfilter_create:

• CustomMemoryAllocator struct in db/c.cc inheriting MemoryAllocator
• Function pointers: allocate, deallocate, usable_size (optional), name (optional)
• state + destructor for user-managed lifetime
• Wrapped in std::shared_ptr via existing rocksdb_memory_allocator_t::rep

Changes

• include/rocksdb/c.h: Add rocksdb_memory_allocator_create() declaration
• db/c.cc: Add CustomMemoryAllocator proxy class and factory function
• db/c_test.c: Add test creating a custom allocator with malloc/free callbacks, wiring it to an LRU cache
• unreleased_history/public_api_changes/custom_memory_allocator.md: Release note

Test Plan

• Added StartPhase("custom_memory_allocator") test in db/c_test.c
• Test creates allocator → sets on LRU cache options → creates cache → destroys in order
• Verifies no crashes or leaks through the callback dispatch path

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