Support resetting custom dimensions and related enhancements

[Stephen-Bao]

Related issue:

• https://github.com/awslabs/aws-embedded-metrics-java/issues/35
• https://github.com/awslabs/aws-embedded-metrics-java/pull/105
• https://github.com/awslabs/aws-embedded-metrics-python/issues/87
• https://github.com/awslabs/aws-embedded-metrics-dotnet/issues/34

This is the equivalent enhancements related to resetting custom dimensions in the Node library. Similar APIs and features should be implemented in all libraries for consistency. Specifially, the enhancements include:

• Provide a resetDimensions(useDefault: boolean) API for resetting dimensions
• Provide a feature flag to control whether flush() should preserve custom dimensions after each execution
• Provide a setDimensions(useDefault: boolean, ...dimensionSets: Array<Record<string, string>>) API to control the behavior of default dimensions while setting new dimensions.