Move SocketAsyncEngine to CoreLib so different SafeHandle types can use it.

[tmds]

Contributes to #47631.

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[rzikm]

What does moving this code achieve? I understand that it is a good step towards merging the engine to threadpool, I am just trying to gauge the benefit this change alone would bring if we were to squeeze it in .NET 11.

Unless we have have other areas that would immediately benefit from this move, I don't see the benefit of rushing this.

[tmds]

I don't mean to squeeze/rush.

This is the first step which unblocks/enables other areas to adopt at their own pace.

I don't think these changes are risky or complex. We have discussed and worked out the API. You can consider to include this in .NET 11 if you have time to review it. Otherwise, you can postpone to .NET 12.