feat: add AsNativeArray() read‑only accessor to NetworkList<T>

[EmandM]

Continues: #3562 from @harayuu9

This PR exposes the contents of a NetworkList<T> as a read‑only NativeArray<T>.
It addresses the recurring need to interoperate efficiently with Burst‑compiled or IJob‑based code without paying the cost of element‑wise copies or allocations.
The accessor simply forwards m_List.AsReadOnly(), preserving zero‑allocation semantics while respecting NativeArray’s lifetime constraints (call‑site must guarantee the list is not mutated or disposed).
No behavioural changes are introduced—only a pure additive surface‑area expansion.

Changelog

• Added: NetworkList<T>.AsNativeArray() to return the list contents as a NativeArray<T>.ReadOnly.

Testing and Documentation

• No tests have been added.
• Includes documentation for the newly added public API function.

Backport

This adds new API surface area and so no backport is required.

[NoelStephensUnity]

@harayuu9
Thank you for your contribution.
As a side note, you can also just do this to accomplish the same thing:

    private NetworkVariable<NativeList<int>> m_MyList = new NetworkVariable<NativeList<int>>();
    public NativeArray<int>.ReadOnly MyList => m_MyList.Value.AsReadOnly();

[NoelStephensUnity]