Add profiling counts to pipeline uses

[walley892]

This PR adds functionality that logs uses of pipeline variants during render passes across all rendering backends for impeller. It also adds a method to the engine service protocol that emits these usage statistics. New functionality is unit tested.

Fixes #176660.

[gemini-code-assist]

Code Review

This PR adds profiling for pipeline variant usage in Impeller. It introduces a mechanism to track pipeline creation and usage, and exposes this data through a new service protocol extension. The changes span across the rendering backends (Metal, GLES, Vulkan) to log pipeline usage, and modify the shell to handle the new service protocol request.

My review focuses on correctness, thread safety, and code improvements. I've found a critical data race in PipelineLibrary due to unsynchronized access to the usage counts map from multiple threads. I've also identified a typo in the new service protocol extension name, and suggested some code simplifications and optimizations. Please see the detailed comments.

[chinmaygarde]

Apologies for the late review! Some minor suggestions.

[auto-submit]

autosubmit label was removed for flutter/flutter/179374, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.