Add Logger to every operation.

[adam-fowler]

Integrate with swift-log and include optional Logger in every operation. With the next release of AsyncHTTPClient we can pass this logger to the HTTPClient. See swift-server/async-http-client#227

Question: What do we want AWSSDKSwift to log?

• Errors returned from AWS
• Warning whenever we don't have credentials
• Info on retrying a failed request
• Info credential updates
• Info credential expiring
• Info on EC2 indicate whether IMSv2 or v1 was used when updating credentials
• Error on syncShutdown

[fabianfett]

if a logger is passed and we receive an error, let's log the error (log level error).
we should log a warning, whenever we don't have credentials.

[adam-fowler]

Info on retries
If AsyncHTTPClient is logging every request we probably don't need to log it as well.
Credential updates, where they were received from, expiring credentials etc

[fabianfett]

Replace the print here.

    deinit {
        // if httpClient was created by AWSClient then it is required to shutdown the httpClient
        if case .createNew = httpClientProvider {
            do {
                try httpClient.syncShutdown()
            } catch {
                print("Error shutting down HTTP client: \(error)")
            }
        }
    }