fix(client): Prevent disposing of the HttpMessageHandler in CamundaCoudTokenProvider

[terjeinnerdal]

Creates the HttpClient in CamundaCloudTokenProvider constructor. Added CamundaCloudTokenProvider.SetHttpMessageHandler method to set the HttpMessageHandler for testing which will create a new HttpClient.

closes #218

[ChrisKujawa]

Hey @terjeinnerdal thanks for fixing this. Could we add a separate test which fails before and now succeeds with the change?

[terjeinnerdal]

Yes, that sounds like a good idea. I'll see if I can find the time today.

[terjeinnerdal]

I've included a test now, but the continuous-integration check fails on the integration tests. It shows the following warning:
"toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
TearDown failed for test fixture Client.IntegrationTests.WorkflowInstanceTest
Docker.DotNet.DockerContainerNotFoundException : Docker API responded with status code=NotFound, response={"message":"No such image: camunda/zeebe:0.23.0"}"

[ChrisKujawa]

Thanks for the fix and the additional test 👍 I think we can simplify the test and then we can merge it :)

[ChrisKujawa]

Suggested change

	var firstToken = await TokenProvider.GetAccessTokenForRequestAsync();
	await TokenProvider.GetAccessTokenForRequestAsync();

[ChrisKujawa]

Suggested change

	var files = Directory.GetFiles(TokenStoragePath);
	var tokenFile = files[0];
	await File.WriteAllTextAsync(tokenFile, "FILE_TOKEN");

[ChrisKujawa]

Suggested change

Token = "SECOND_TOKEN";

[ChrisKujawa]

Thank you very much for your contribution @terjeinnerdal I created a separate PR to do the clean up for you and to merge it with the latest changes. #235