Add pin rows to Profile Memory table

[khanak0509]

Fixes #8898

Trace Instances already keeps traced classes at the top (via the checkbox), but Profile Memory didn’t have anything similar

This PR adds a Pin column to Profile Memory. Tap the pin icon on a row and that class stays in the pinned section at the top (under All Classes), with a divider between pinned and the rest of the table, same general idea as Trace Instances, but without turning on allocation tracing

before

after

Pre-launch Checklist

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Issues checklist

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Tests checklist

• I added new tests to check the change I am making...
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AI-tooling checklist

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Feature-change checklist

• This PR does not change the DevTools UI or behavior and...
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• OR this PR does change the DevTools UI or behavior and...
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  • I included before/after screenshots and/or a GIF demo of the new UI to my PR description.
  • I ran the DevTools app locally to manually verify my changes.

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[gemini-code-assist]

Code Review

This pull request introduces a feature to pin classes to the top of the Profile Memory table in DevTools. The implementation involves updating the AdaptedProfile and ProfileRecord models to track pinning status, adding pinning logic and serialization to the ProfilePaneController, and creating a new _PinColumn for the UI. Review feedback identifies a logical issue where the pinned classes listener is not updated during initialization, suggests improving the stateful widget's resilience to controller changes, and highlights performance concerns regarding redundant builders and inefficient key generation for the table.