Add support for getting a string representation of the LayerTree

[gw280]

Description

This adds a method (based on getRenderTree) which allows developers to programmatically get a string representation of the layer tree using flutter_driver.

Related Issues

#46992

Tests

I added the following tests:

flutter_driver_test.dart: getLayerTree

Checklist

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• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
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Breaking Change

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• Yes, this is a breaking change. If not, delete the remainder of this section.
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[cbracken]

[dnfield]

LGTM with comment