Use botocore.UNSIGNED signature version with custom config

[dominictarro]

The "signature_version" argument of botocore.config.Config accepts strings for signers and the botocore.UNSIGNED singleton when a signed should not be configured.

Change allows users to pass the AWSCredentials configuration {"signature_version": "unsigned"} and prefect_aws.ClientParameters.get_params_override will convert it to the singleton.

Also changed TestAwsClientParameters.test_with_custom_config test argument for ClientParameters.config to a dictionary since that is what it will receive from Orion.

This should probably be handled within botocore, but it's a quick patch for accessing some public buckets.

Closes #219

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[desertaxle]

Thanks for the contribution! I have one nit on the changelog; otherwise, this looks good!

[zanieb]

Thanks!