Fix StatsWrapper generated output path

[mavam]

Summary:

• Generate StatsWrapper.h under PROXYGEN_GENERATED_ROOT, matching the declared custom command output.
• Create the stats output directory before running the generator.
• Track gen_StatsWrapper.sh as an input and use VERBATIM for the command.

The generator writes to $1/proxygen/lib/stats/StatsWrapper.h, but the CMake rule passed PROXYGEN_FBCODE_ROOT while declaring the output below PROXYGEN_GENERATED_ROOT. This makes the generated output not match the declared custom command output.

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