Add missing Dispose() calls

[brianpursley]

While not recommended, if you repeatedly create a Kubernetes client inside of a loop, there appears to be a memory leak, even if you dispose it.

This PR adds the following to fix this:

• Adds .Dispose() for FirstMessageHandler
• Adds .Dispose() for HttpClientHandler
• A few other minor changes in the Dispose() method
• Adds Using for httpRequest (while unrelated to the Kubernetes client lifecycle, I believe this should also be disposed)

Possible fix for #1539 (I suspect that issue is not related only to aot).

Test Results

I tested this using the following console program and running in Rider using memory profiling (sampled allocations):

using k8s;

var config = KubernetesClientConfiguration.BuildConfigFromConfigFile();

for (var i = 0; i < 1000; i++)
{
    using var client = new Kubernetes(config);
    var list = client.CoreV1.ListNamespacedPod("kube-system");
    Console.WriteLine(string.Join(", ", list.Items.Select(x => x.Metadata.Name)));
}

Before the fix

Memory increases slowly, eventually reaching ~163MB by the end of the program:

After the fix

Memory holds stead around ~123MB:

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[brianpursley]

Hmm, i think I can't do this:

using var httpRequest = new HttpRequestMessage

... because this is not awaited ...

return SendRequestRaw("", httpRequest, cancellationToken);

So the using disposes httpRequest before SendRequestRaw has executed.

SendRequest should probably use async/await, but I think that is a different problem. I will update this PR to remove that change.

[brianpursley]

OK, removed the using change. I ran the profile again and it was unaffected, so maybe that change doesn't really make a difference anyway.

[tg123]

Good catch

/LGTM

[codecov-commenter]

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Additional details and impacted files

@@            Coverage Diff            @@
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  Files             ?      103           
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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brianpursley, tg123

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