Add enum + event constructor for code size analysis

[eliasyishak]

Related to tracker issue:

• ☂️ package:unified_analytics implementation in flutter tool flutter/flutter#128251

This migrates the CodeSizeEvent event, which is defined here and below

/// An event that reports when the code size measurement is run via `--analyze-size`.
class CodeSizeEvent extends UsageEvent {
  CodeSizeEvent(String platform, {
    required Usage flutterUsage,
  }) : super(
    'code-size-analysis',
    platform,
    flutterUsage: flutterUsage
  );
}

There is only one reference of this class getting called within the flutter tool code base and it is within flutter_tools/lib/src/base/analyze_size.dart

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:unified_analytics	5.4.0	ready to publish	unified_analytics-v5.4.0
package:graphs	2.3.2-wip	WIP (no publish necessary)
package:cli_config	0.1.2	already published at pub.dev
package:extension_discovery	2.0.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[christopherfujino]

LGTM