feat(amplify_event_enrichment): add example app

[ekjotmultani]

Description

Adds the example Flutter app for the amplify_event_enrichment package, demonstrating event enrichment client usage with session tracking, global fields, and lifecycle observation.

This PR is stacked on #7029 (core package implementation). Review #7029 first.

Structure (3-way split)

PR	Branch	Content
#7029	feat/event-enrichment-client	Core package (lib only)
This PR	feat/event-enrichment-client-example	Example app (92 files)
#7068	feat/event-enrichment-client-tests	Unit tests (5 files)

What's included

• Full Flutter example app with Android, iOS, and macOS targets
• Demonstrates EventEnrichmentClient initialization and usage
• Shows session management, global fields, and lifecycle observation

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.23%. Comparing base (b05a0ad) to head (14c3d90).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feat/event-enrichment-client    #7076   +/-   ##
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  Coverage                         43.23%   43.23%           
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  Files                                99       99           
  Lines                              7769     7769           
  Branches                           3401     3401           
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  Hits                               3359     3359           
  Misses                             4410     4410           

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[cadivus]

In example apps for our libs in this repo, I'd use any as a version so that we don't need to care about version bumps.