Implement IAsyncDisposable on AdminClient

[verdie-g]

What

AdminClient.Dispose does sync-over-async which impacts the throughput of the thread pool. There is a comment on the method that says

In the current implementation, this method may block for up to 100ms. This will be replaced with a non-blocking version in the future.

This change fixes that by implementing IAsyncDisposable.

The AdminClient class partially implemented a dispose pattern (Dispose(bool) + GC.SuppressFinalize(this)) but it was missing a finalizer. Anyway, this class doesn't own any native resources so this is not needed.

Checklist

• Contains customer facing changes? A new method is added to IAdminClient
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

[Havret]

This implementation needs some adjustments.

[Havret]

This class owns native resources (see the DisposeResources method), including:
• handle
• resultQueue
• ownedClient (optional)

[verdie-g]

This class owns native resources (see the DisposeResources method), including:
• handle
• resultQueue
• ownedClient (optional)

• handle doesn't seem to be owned by the AdminClient
• ownedClient is managed, it may own some unmanaged fields, but it's its own responsibility to dispose them in its finalizer
• resultQueue is indeed unmanaged, I missed that one. So I completed the dispose pattern following https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync