Implement CloudWatch Metrics around Handler invocation

[rjlohan]

Inside the Lambda runtime wrapper, we will want to push metrics to CloudWatch in the owning account. The metrics we publish should include;

• handler invocation start/end times
• number of retry callbacks
• errors/exceptions

[rjlohan]

Standard metrics are now emitted to CloudWatch by the https://github.com/aws-cloudformation/aws-cloudformation-rpdk-java-plugin/blob/master/src/main/java/com/aws/cfn/LambdaWrapper.java

An instance of MetricsPublisher is passed in and injected with a default CloudWatch metrics proxy