Allow credentials to be specified

[attila]

It would be really nice if we could configure the CloudWatchLogs instance access not only with access keys but by passing in an AWS.Credentials instance to the CloudWatchLogs constructor. We have a use case where a set of desktop applications stream logs via an assumed role. This role is used throughout the application and it's assumed via AWS.ChainableTemporaryCredentials so that the SDK can automatically refresh the session token whenever necessary.

I saw that configuration only checks for adds accessKeyId and secretAccessKey which covers the most basic use case but unfortunately it's not sufficient for more complex IAM setups