Support resetting custom dimensions and related enhancements

[Stephen-Bao]

Related issue:

• Support Clearing Custom Dimensions on MetricsLogger aws-embedded-metrics-java#35
• Add support to clear custom dimensions and configure flush behavior on dimensions aws-embedded-metrics-java#105
• Support resetting custom dimensions and related enhancements aws-embedded-metrics-node#132
• Support resetting custom dimensions and related enhancements aws-embedded-metrics-dotnet#34

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

• Provide a reset_dimensions(useDefault: bool) API for resetting dimensions
• Provide a feature flag to control whether flush() should preserve custom dimensions after each execution
• Provide a set_dimensions(useDefault: bool, *dimensions: Dict[str, str]) API to control the behavior of default dimensions while setting new dimensions.