External allocators

[dallasmarlow]

feat: Add support for custom external allocators

This pull request adds support for third-party allocators by introducing a new ExternalAllocator trait.

Motivation

The existing Allocator trait requires DerefMut<Target=[u8]>, which is incompatible with certain allocator designs, such as arena allocators that do not expose their memory as a single slice. This change introduces a reallocate/deallocate model to support these use cases.

Implementation

This feature is implemented as a non-breaking addition to the existing allocation system.

• ExternalAllocator Trait: A new pub unsafe trait ExternalAllocator is defined, which requires reallocate and deallocate methods.
• AllocatorAdapter Struct: A generic struct that implements the Allocator trait by wrapping a type E that implements ExternalAllocator.
• It uses static dispatch by being generic over the allocator type, which avoids runtime vtable lookups.
• A Drop implementation on the adapter calls the deallocate method to ensure managed resources are correctly released.
• Constructor: The AllocatorAdapter::new_builder function is added to construct a FlatBufferBuilder with a given custom allocator.

Validation

This change is backward-compatible. All existing tests pass, and a new integration test using bumpalo as a dev-dependency has been added to verify the functionality of the ExternalAllocator implementation.

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[dallasmarlow]

After a closer look, this PR is not ready for review yet and needs further work. Moved to a draft for now.