Support clear custom dimensions and related enhancements

[Stephen-Bao]

Issue #, if available:
#87

Description of changes:
Added support to clear custom dimensions, configure the behavior of flush on custom dimensions, and related enhancement. This is the equivalent feature implementation as in the Java library:

• Add support to clear custom dimensions and configure flush behavior on dimensions aws-embedded-metrics-java#105

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[hussam789]

PR Code Suggestions ✨

Category	Suggestion	Impact
Possible issue	Make parameter keyword-only The use_default parameter should be marked as keyword-only to match the documentation in README.md which states it's a "keyword-only parameter". Without this, users could accidentally pass a boolean as a positional argument, causing unexpected behavior. aws_embedded_metrics/logger/metrics_logger.py [68-70] -def set_dimensions(self, *dimensions: Dict[str, str], use_default: bool = False) -> "MetricsLogger": +def set_dimensions(self, *dimensions: Dict[str, str], *, use_default: bool = False) -> "MetricsLogger": self.context.set_dimensions(list(dimensions), use_default) return self Apply this suggestion Suggestion importance[1-10]: 7 __ Why: The suggestion correctly updates the function signature to enforce the keyword-only nature of the 'use_default' parameter, aligning it with the README.md documentation. This helps prevent unintended behavior from passing a boolean as a positional argument.	Medium
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