Add request/response traces

[jdisanti]

This adds request/response tracing called out in #371.

Example output:

May 20 08:53:31.506 TRACE send_operation{operation="PutParameter" service="ssm"}: smithy_http_tower::dispatch: request=Request { method: POST, uri: https://ssm.us-east-1.amazonaws.com/, version: HTTP/1.1, headers: {"content-type": "application/x-amz-json-1.1", "x-amz-target": "AmazonSSM.PutParameter", "content-length": "117", "host": "ssm.us-east-1.amazonaws.com", "authorization": "<redacted>", "x-amz-date": "20210520T155331Z", "x-amz-security-token": "<redacted>", "user-agent": "aws-sdk-rust/0.1.0 os/linux lang/rust/1.52.1", "x-amz-user-agent": "aws-sdk-rust/0.1.0 api/ssm/0.0.2 os/linux lang/rust/1.52.1"}, body: SdkBody(Once(Some(b"{\"Name\":\"test_parameter_name\",\"Description\":\"some description\",\"Value\":\"some_value\",\"Type\":\"String\",\"Overwrite\":true}"))) }

May 20 08:53:37.051 TRACE send_operation{operation="PutParameter" service="ssm"}:load_response: smithy_http::middleware: response=Response { status: 200, version: HTTP/1.1, headers: {"server": "Server", "date": "Thu, 20 May 2021 15:53:37 GMT", "content-type": "application/x-amz-json-1.1", "content-length": "32", "connection": "keep-alive", "x-amzn-requestid": "913ce42a-a071-47b6-ba6b-7d1f0b8d6f1a"}, body: SdkBody(Streaming(Body(Streaming))) }

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[rcoh]

LGTM, would be nice to remove this box at some point but one extra allocation per request isn't really hurting anyone.