Use `externalSize` of `NativeFinalizer.attach` to help dart VM GC scheduling heuristics

[rainyl]

Is your feature request related to a problem? Please describe.

Currently the disposing of native objects is not smart enough and relies on manually disposing, adding the externalSize optional parameter can help this.

Describe the solution you'd like

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Additional context

• https://api.dart.dev/stable/3.7.0/dart-ffi/NativeFinalizer/attach.html