Wasmtime: generalize async_stack_zeroing knob to cover initialization

[fitzgen]

This commit moves the knob from the PoolingInstanceAllocatorConfig to the regular Config and now controls both whether stacks are zeroed before reuse and whether they are zeroed before the initial use. The latter doesn't matter usually, since anonymous mmaps are already zeroed so we don't have to do anything there, but for no-std environments it is the difference between manually zeroing the stack or simply using unininitialized memory.

[github-actions]

Label Messager: wasmtime:config

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